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/finddebugbinary.c

@@ -22,46 +22,37 @@
 #include "libc/runtime/symbols.internal.h"
 #include "libc/str/str.h"
 
-static char *g_comdbg;
-static char g_comdbg_buf[PATH_MAX + 1];
-
-static optimizesize textstartup void g_comdbg_init() {
-  char *p;
-  size_t n;
-  static bool once;
-  if (!once) {
-    if (!(g_comdbg = getenv("COMDBG"))) {
-      p = program_executable_name;
-      n = strlen(p);
-      if (n > 4 && READ32LE(p + n - 4) == READ32LE(".dbg")) {
-        g_comdbg = p;
-      } else if (n > 4 && READ32LE(p + n - 4) == READ32LE(".com") &&
-                 n + 4 <= PATH_MAX) {
-        mempcpy(mempcpy(g_comdbg_buf, p, n), ".dbg", 5);
-        if (fileexists(g_comdbg_buf)) {
-          g_comdbg = g_comdbg_buf;
-        }
-      } else if (n + 8 <= PATH_MAX) {
-        mempcpy(mempcpy(g_comdbg_buf, p, n), ".com.dbg", 9);
-        if (fileexists(g_comdbg_buf)) {
-          g_comdbg = g_comdbg_buf;
-        }
-      }
-    }
-    once = true;
-  }
-}
-
 /**
  * Returns path of binary with the debug information, or null.
  *
  * @return path to debug binary, or NULL
  */
 const char *FindDebugBinary(void) {
-  g_comdbg_init();
-  return g_comdbg;
+  static bool once;
+  static char *res;
+  static char buf[PATH_MAX + 1];
+  char *p;
+  size_t n;
+  if (!once) {
+    if (!(res = getenv("COMDBG"))) {
+      p = program_executable_name;
+      n = strlen(p);
+      if (n > 4 && READ32LE(p + n - 4) == READ32LE(".dbg")) {
+        res = p;
+      } else if (n > 4 && READ32LE(p + n - 4) == READ32LE(".com") &&
+                 n + 4 <= PATH_MAX) {
+        mempcpy(mempcpy(buf, p, n), ".dbg", 5);
+        if (fileexists(buf)) {
+          res = buf;
+        }
+      } else if (n + 8 <= PATH_MAX) {
+        mempcpy(mempcpy(buf, p, n), ".com.dbg", 9);
+        if (fileexists(buf)) {
+          res = buf;
+        }
+      }
+    }
+    once = true;
+  }
+  return res;
 }
-
-const void *const g_comdbg_ctor[] initarray = {
-    g_comdbg_init,
-};