Fix XNU / FreeBSD / OpenBSD / RHEL5 / NT bugs

For the first time ever, all tests in this codebase now pass, when
run automatically on macos, freebsd, openbsd, rhel5, rhel7, alpine
and windows via the network using the runit and runitd build tools

- Fix vfork exec path etc.
- Add XNU opendir() support
- Add OpenBSD opendir() support
- Add Linux history to syscalls.sh
- Use copy_file_range on FreeBSD 13+
- Fix system calls with 7+ arguments
- Fix Windows with greater than 16 FDs
- Fix RUNIT.COM and RUNITD.COM flakiness
- Fix OpenBSD munmap() when files are mapped
- Fix long double so it's actually long on Windows
- Fix OpenBSD truncate() and ftruncate() thunk typo
- Let Windows fcntl() be used on socket files descriptors
- Fix Windows fstat() which had an accidental printf statement
- Fix RHEL5 CLOCK_MONOTONIC by not aliasing to CLOCK_MONOTONIC_RAW

This is wonderful. I never could have dreamed it would be possible
to get it working so well on so many platforms with tiny binaries.

Fixes #31
Fixes #25
Fixes #14

libc/sysv/systemfive.S

@@ -68,19 +68,27 @@ __hostos:
 /	Performs System Five System Call.
 /
 /	Cosmopolitan is designed to delegate all function calls into the
-/	Linux, FreeBSD, OpenBSD, and XNU kernels through this function,
-/	with few exceptions. This function is intended to be called via
+/	Linux, FreeBSD, OpenBSD, and XNU kernels via this function, with
+/	few exceptions. This function should generally only be called by
 /	generated thunks in the libc/sysv/syscalls/ directory.
 /
-/	It's safe to call this function on Windows, where it'll always
-/	return -1 w/ errno == ENOSYS. Further note that -1 is the only
+/	It's safe to call this function on Windows, where it will always
+/	return -1 with errno == ENOSYS. Further note that -1 is the only
 /	return value that means error, a common anti-pattern is to check
 /	for values less than 0 (which is more problematic on 32-bit).
 /
-/	It's important to consider that system calls are an order of a
+/	It is important to consider that system calls are one order of a
 /	magnitude more expensive than normal function calls. For example
 /	getpid() on Linux usually takes 500ns, and cached i/o calls will
-/	take 1µs or more.
+/	take 1µs or more. So we don't need to inline them like Chromium.
+/
+/	Another thing to consider is that BSDs only loosely follow the
+/	System Five ABI for the SYSCALL instruction. For example Linux
+/	always follows the six argument limit but the FreeBSD sendfile
+/	system call accepts a seventh argument that is passed on stack
+/	and OpenBSD modifies functions like mmap so that the sixth arg
+/	is passed on the stack. There's also the carry flag convention
+/	that XNU, FreeBSD, and OpenBSD inherited from 386BSD aka Jolix
 /
 /	@param	%rax function ordinal supplied by jump slot
 /	@param	%rdi,%rsi,%rdx,%rcx,%r8,%r9 and rest on stack
@@ -93,16 +101,16 @@ __systemfive:
 	.previous
 .Lanchorpoint:
 systemfive.linux:
-	movswl	%ax,%eax
-	test	%eax,%eax
-	js	systemfive.enosys
-	mov	%rcx,%r10
-	push	%rbp
-	mov	%rsp,%rbp
-	syscall
-	pop	%rbp
-	cmp	$-4095,%rax
-	jae	systemfive.error
+	movswl	%ax,%eax		# gnu/systemd ordinal is first word
+	test	%eax,%eax		# sign extend to optimize code size
+	js	systemfive.enosys	# test for syscalls.sh ordinal ffff
+	mov	%rcx,%r10		# syscall instruction clobbers %rcx
+	push	%rbp			# linux never reads args from stack
+	mov	%rsp,%rbp		# having frame will help backtraces
+	syscall				# this is known as a context switch
+	pop	%rbp			# next we check to see if it failed
+	cmp	$-4095,%rax		# system five nexgen32e abi § A.2.1
+	jae	systemfive.error	# encodes errno as neg return value
 	ret
 	.endfn	systemfive.linux,globl,hidden
 systemfive.error:
@@ -110,10 +118,9 @@ systemfive.error:
 /	𝑠𝑙𝑖𝑑𝑒
 	.endfn	systemfive.error,globl,hidden
 systemfive.errno:
-	mov	%eax,errno(%rip)
-	push	$-1
-	pop	%rax
-	stc
+	mov	%eax,errno(%rip)	# normalize to c library convention
+	push	$-1			# negative one is only error result
+	pop	%rax			# the push pop is to save code size
 	ret
 	.endfn	systemfive.errno,globl,hidden
 systemfive.enosys:
@@ -132,20 +139,16 @@ systemfive.freebsd:
 systemfive.bsd:
 	cmp	$0xfff,%ax
 	jae	systemfive.enosys
-	mov	%rcx,%r10
-	push	%rbp
-	mov	%rsp,%rbp
-	syscall
-	pop	%rbp
-	jc	systemfive.errno
+	mov	%rcx,%r10		# note: we do not create a stack frame
+	syscall				# bsd will need arg on stack sometimes
+	jc	systemfive.errno	# bsd sets carry flag if %rax is errno
 	ret
 	.endfn	systemfive.bsd
 systemfive.xnu:
-/	do this to rax
-/	0x????????2153????
+/	0x????????2153????		# how syscalls.sh encodes xnu ordinals
 /	          │└┴┴─┐
 /	          └┐   ├┬┐
-/	0x0000000002000153
+/	0x0000000002000153		# how xnu wants ordinals to be encoded
 	mov	%eax,%r11d
 	shr	$4*7,%r11d
 	shl	$4*6,%r11d