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/stdio/fgets_test.c

@@ -16,9 +16,10 @@
 │ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
-#include "libc/mem/mem.h"
 #include "libc/mem/gc.internal.h"
+#include "libc/mem/mem.h"
 #include "libc/stdio/stdio.h"
+#include "libc/str/str.h"
 #include "libc/testlib/ezbench.h"
 #include "libc/testlib/hyperion.h"
 #include "libc/testlib/testlib.h"
@@ -33,6 +34,18 @@ TEST(fgets, test) {
   fclose(f);
 }
 
+TEST(fgets, eof) {
+  FILE *f;
+  char s[10];
+  char buf[] = "test";
+  ASSERT_NE(NULL, (f = fmemopen(buf, sizeof buf, "r")));
+  ASSERT_EQ(s, fgets(s, sizeof s, f));
+  ASSERT_EQ(0, strcmp(s, buf));
+  ASSERT_EQ(0, fgets(s, sizeof s, f));
+  ASSERT_EQ('t', s[0]);
+  ASSERT_EQ(0, fclose(f));
+}
+
 void Benchmark(void) {
   FILE *f;
   char *line;