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/runtime/vfork.S

@@ -17,14 +17,10 @@
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/dce.h"
+#include "libc/calls/strace.internal.h"
 #include "libc/macros.internal.h"
 .privileged
 
-#ifdef __FSANITIZE_ADDRESS__
-vfork:	jmp	fork			# TODO: asan and vfork don't mix?
-	.endfn	vfork,globl
-#else
-
 //	Forks process without copying page tables.
 //
 //	This is the same as fork() except it's optimized for the case
@@ -40,11 +36,24 @@ vfork:	jmp	fork			# TODO: asan and vfork don't mix?
 //	@returnstwice
 //	@vforksafe
 vfork:
+#ifdef __FSANITIZE_ADDRESS__
+	jmp	fork			# TODO: asan and vfork don't mix?
+	.endfn	vfork,globl
+#else
 #if SupportsWindows()
 	testb	IsWindows()
-	jnz	sys_fork_nt
+	jnz	sys_fork_nt		# and we're lucky to have that
 #endif
+#if SupportsOpenbsd()
+	testb	IsOpenbsd()
+	jnz	fork			# fake vfork plus msyscall issues
+#endif
+#ifdef SYSDEBUG
+	ezlea	.Llog,di
+	call	__stracef
+#endif /* SYSDEBUG */
 	mov	__NR_vfork(%rip),%eax
+	mov	errno(%rip),%r8d	# avoid question of @vforksafe errno
 	pop	%rsi			# saves return address in a register
 #if SupportsBsd()
 	testb	IsBsd()
@@ -56,7 +65,8 @@ vfork:
 	cmp	$-4095,%eax
 	jae	systemfive_error
 #endif
-0:	ezlea	__vforked,di
+0:	mov	%r8d,errno(%rip)
+	ezlea	__vforked,di
 	test	%eax,%eax
 	jz	1f
 	decl	(%rdi)
@@ -81,4 +91,11 @@ vfork.bsd:
 	.endfn	vfork.bsd
 #endif /* BSD */
 
+#ifdef SYSDEBUG
+	.rodata.str1.1
+.Llog:	.ascii	STRACE_PROLOGUE
+	.asciz	"vfork()%n"
+	.previous
+#endif /* DEBUGSYS */
+
 #endif /* __FSANITIZE_ADDRESS__ */