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/intrin/somanyasan.S

@@ -184,7 +184,7 @@ __asan_version_mismatch_check_v8:
 	.endfn	__asan_version_mismatch_check_v8,globl
 
 //	Initializes Address Sanitizer runtime earlier if linked.
-	.init.start 301,_init_asan
+	.init.start 303,_init_asan
 	push	%rdi
 	push	%rsi
 	mov	%r12,%rdi
@@ -194,7 +194,7 @@ __asan_version_mismatch_check_v8:
 	call	__asan_init
 	pop	%rsi
 	pop	%rdi
-	.init.end 301,_init_asan
+	.init.end 303,_init_asan
 
 __asan_before_dynamic_init:
 	push	%rbp