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/unicode/unicode-properties.txt

@@ -28,3 +28,10 @@ Cf = Other, format
 Cs = Other, surrogate
 Co = Other, private use
 Cn = Other, not assigned (including noncharacters)
+
+W  Wide      Naturally wide character, e.g. Hiragana.
+Na Narrow    Naturally narrow character, e.g. ISO Basic Latin alphabet.
+F  Fullwidth Wide variant with compatibility normalisation to naturally narrow character, e.g. fullwidth Latin script.
+H  Halfwidth Narrow variant with compatibility normalisation to naturally wide character, e.g. half-width kana. Includes U+20A9 (₩) as an exception.
+A  Ambiguous Characters included in East Asian DBCS codes but also in European SBCS codes, e.g. Greek alphabet. Duospaced behaviour can consequently vary.
+N  Neutral   Characters which do not appear in East Asian DBCS codes, e.g. Devanagari.