Improve cosmo's conformance to libc-test

This change addresses various open source compatibility issues, so that
we pass 313/411 of the tests in https://github.com/jart/libc-test where
earlier today we were passing about 30/411 of them, due to header toil.
Please note that Glibc only passes 341/411 so 313 today is pretty good!

- Make the conformance of libc/isystem/ headers nearly perfect
- Import more of the remaining math library routines from Musl
- Fix inconsistencies with type signatures of calls like umask
- Write tests for getpriority/setpriority which work great now
- conform to `struct sockaddr *` on remaining socket functions
- Import a bunch of uninteresting stdlib functions e.g. rand48
- Introduce readdir_r, scandir, pthread_kill, sigsetjmp, etc..

Follow the instructions in our `tool/scripts/cosmocc` toolchain to run
these tests yourself. You use `make CC=cosmocc` on the test repository

libc/fmt/strtoul.c

@@ -19,6 +19,7 @@
 #include "libc/errno.h"
 #include "libc/fmt/conv.h"
 #include "libc/fmt/strtol.internal.h"
+#include "libc/limits.h"
 #include "libc/str/str.h"
 #include "libc/str/tab.internal.h"
 
@@ -34,6 +35,7 @@
  *     on the the prefixes 0 (octal), 0x (hexadecimal), 0b (binary), or
  *     decimal (base 10) by default
  * @return decoded integer mod 2⁶⁴ negated if leading `-`
+ * @raise ERANGE on overflow
  */
 unsigned long strtoul(const char *s, char **endptr, int base) {
   char t = 0;
@@ -45,8 +47,12 @@ unsigned long strtoul(const char *s, char **endptr, int base) {
   if ((c = kBase36[c & 255]) && --c < base) {
     t |= 1;
     do {
-      x *= base;
-      x += c;
+      if (__builtin_mul_overflow(x, base, &x) ||
+          __builtin_add_overflow(x, c, &x)) {
+        if (endptr) *endptr = s + 1;
+        errno = ERANGE;
+        return ULONG_MAX;
+      }
     } while ((c = kBase36[*++s & 255]) && --c < base);
   }
   if (t && endptr) *endptr = s;