Introduce --strace flag for system call tracing

This is similar to the --ftrace (c function call trace) flag, except
it's less noisy since it only logs system calls to stderr. Having this
flag is valuable because (1) system call tracing tells us a lot about
the behavior of complex programs and (2) it's usually very hard to get
system call tracing on various operating systems, e.g. strace, ktrace,
dtruss, truss, nttrace, etc. Especially on Apple platforms where even
with the special boot trick, debuggers still aren't guaranteed to work.

    make -j8 o//examples
    o//examples/hello.com --strace

This is enabled by default in MODE=, MODE=opt, and MODE=dbg. In MODE=dbg
extra information will be printed.

    make -j8 MODE=dbg o/dbg/examples
    o/dbg/examples/hello.com --strace |& less

This change also changes:

- Rename IsText() → _istext()
- Rename IsUtf8() → _isutf8()
- Fix madvise() on Windows NT
- Fix empty string case of inet_ntop()
- vfork() wrapper now saves and restores errno
- Update xsigaction() to yoink syscall support

libc/sysv/systemfive.S

@@ -16,13 +16,14 @@
 │ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/calls/strace.internal.h"
 #include "libc/dce.h"
-#include "libc/sysv/consts/prot.h"
-#include "libc/sysv/consts/nr.h"
-#include "libc/sysv/consts/map.h"
 #include "libc/macros.internal.h"
-#include "libc/sysv/consts/prot.h"
 #include "libc/nexgen32e/macros.h"
+#include "libc/sysv/consts/map.h"
+#include "libc/sysv/consts/nr.h"
+#include "libc/sysv/consts/prot.h"
+#include "libc/sysv/consts/prot.h"
 
 /*                                           ▄▄▄
                        ▄▄▄                    ▀▓▓▒▄
@@ -306,8 +307,29 @@ _init_systemfive_magnums:
 	jnz	3b
 	xchg	%rbx,%rax
 	stosq
+#ifdef SYSDEBUG
+	inc	%r8d
+#endif
 	jmp	2b
-5:	pop	%rdi
+5:	nop
+#ifdef SYSDEBUG
+	push	%rdi
+	push	%rsi
+	push	%r8
+	call	__describe_os
+	mov	%rax,%rdx
+	pop	%r8
+	.weak	__stracef
+	ezlea	__stracef,ax
+	test	%rax,%rax
+	jz	5f
+	ezlea	.Llog,di
+	mov	%r8d,%esi
+	call	*%rax
+5:	pop	%rsi
+	pop	%rdi
+#endif /* DEBUGSYS */
+	pop	%rdi
 	pop	%rsi
 	pop	%rbx
 //	𝑠𝑙𝑖𝑑𝑒
@@ -325,8 +347,12 @@ _init_systemfive_pid:
 	.endfn	_init_systemfive_pid
 #endif
 #if SupportsSystemv() && !defined(TINY)
-_init_systemfive_stack:					# determinism ftw!
-#if SupportsWindows() || SupportsMetal()
+
+//	Create a stack with deterministic readable addresses.
+//	If ape_execve() already created us a stack that meets
+//	the requirements of STATIC_STACK_SIZE() then we skip.
+_init_systemfive_stack:
+#if SupportsWindows() || SupportsMetal() || SupportsOpenbsd()
 	testb	$WINDOWS|METAL,__hostos(%rip)
 	jnz	_init_systemfive_done
 #endif
@@ -512,6 +538,14 @@ syscon_windows:/*
 	.globl	syscon_windows
 #endif
 
+#ifdef SYSDEBUG
+	.rodata.str1.1
+.Llog:	.ascii	STRACE_PROLOGUE
+	.ascii	"bell system five system call support"
+	.asciz	" %'u magnums loaded on %s%n"
+	.previous
+#endif /* DEBUGSYS */
+
 	.weak	ape_stack_vaddr
 	.weak	ape_stack_memsz
 	.weak	ape_stack_align