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

test/libc/fmt/atoi_test.c

@@ -172,13 +172,6 @@ TEST(strtoul, weirdComma) {
   EXPECT_EQ(0, e - p);
 }
 
-TEST(strtoul, outsideLimit_doesModulus) {
-  EXPECT_EQ(/* python -c 'print(((2**123-1)/123)%(2**64))' */
-            9298358801382050408ull,
-            strtoul(/* python -c 'print((2**123-1)/123)' */
-                    "86453853384384772221385825058884200", 0, 10));
-}
-
 TEST(strtol, testHex) {
   EXPECT_EQ(0, strtol("", 0, 16));
   EXPECT_EQ(0, strtol("0", 0, 16));
@@ -556,6 +549,15 @@ TEST(wcstoumax, testIBM) {
   ASSERT_STREQ(L"f", e);
 }
 
+TEST(strtoul, testoverflow) {
+  errno = 0;
+  char *e = 0;
+  unsigned long x = strtoul("18446744073709551616", &e, 0);
+  ASSERT_EQ(ULONG_MAX, x);
+  ASSERT_EQ(ERANGE, errno);
+  ASSERT_STREQ("", e);
+}
+
 BENCH(atoi, bench) {
   EZBENCH2("atoi 10⁸", donothing, EXPROPRIATE(atoi(VEIL("r", "100000000"))));
   EZBENCH2("strtol 10⁸", donothing,