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/cosmo.S

@@ -21,6 +21,7 @@
 #include "libc/notice.inc"
 #include "libc/sysv/consts/prot.h"
 #include "libc/sysv/consts/map.h"
+#include "libc/calls/strace.internal.h"
 #include "libc/dce.h"
 .text.startup
 
@@ -37,6 +38,10 @@ cosmo:	push	%rbp
 	mov	%rsi,%r13
 	mov	%rdx,%r14
 	mov	%rcx,%r15
+#ifdef SYSDEBUG
+	call	__strace_init
+	mov	%eax,%r12d
+#endif /* SYSDEBUG */
 #ifdef __FAST_MATH__
 	push	%rax
 	stmxcsr	(%rsp)
@@ -72,7 +77,7 @@ cosmo:	push	%rbp
 	.endfn	cosmo,weak
 
 #ifdef __PG__
-	.init.start 800,_init_ftrace
+	.init.start 306,_init_ftrace
 	push	%rdi
 	push	%rsi
 	mov	%r12d,%edi
@@ -86,5 +91,21 @@ cosmo:	push	%rbp
 	mov	%eax,%r12d
 	pop	%rsi
 	pop	%rdi
-	.init.end 800,_init_ftrace
+	.init.end 306,_init_ftrace
+#endif
+
+#if IsModeDbg()
+#ifdef SYSDEBUG
+	.init.start 307,_init_printargs
+	push	%rdi
+	push	%rsi
+	mov	%r12d,%edi
+	mov	%r13,%rsi
+	mov	%r14,%rdx
+	mov	%r15,%rcx
+	call	__printargs
+	pop	%rsi
+	pop	%rdi
+	.init.end 307,_init_printargs
+#endif
 #endif