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

test/libc/sock/inet_ntop_test.c

@@ -41,17 +41,17 @@ TEST(inet_ntop, testBadFamily) {
   uint8_t localhost[4] = {127, 0, 0, 1};
   ASSERT_EQ(NULL, inet_ntop(666, localhost, buf, sizeof(buf)));
   EXPECT_EQ(EAFNOSUPPORT, errno);
-  ASSERT_STREQ("", buf);
+  ASSERT_STREQ("hi", buf);
 }
 
 TEST(inet_ntop, testNoSpace) {
   char *buf = memcpy(malloc(16), "hi", 3);
   uint8_t localhost[4] = {127, 0, 0, 1};
-  ASSERT_EQ(NULL, inet_ntop(AF_INET, localhost, buf, 0));
+  EXPECT_EQ(NULL, inet_ntop(AF_INET, localhost, buf, 1));
   EXPECT_EQ(ENOSPC, errno);
   ASSERT_STREQ("hi", buf);
   ASSERT_EQ(NULL, inet_ntop(AF_INET, localhost, buf, 7));
-  ASSERT_STREQ("", buf);
+  ASSERT_STREQ("hi", buf);
   free(buf);
 }