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/calls/sigaltstack.c

@@ -16,7 +16,6 @@
 │ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
-#include "libc/intrin/strace.internal.h"
 #include "libc/calls/struct/metasigaltstack.h"
 #include "libc/calls/struct/sigaltstack.h"
 #include "libc/calls/struct/sigaltstack.internal.h"
@@ -24,6 +23,8 @@
 #include "libc/dce.h"
 #include "libc/intrin/asan.internal.h"
 #include "libc/intrin/describeflags.internal.h"
+#include "libc/intrin/strace.internal.h"
+#include "libc/sysv/consts/ss.h"
 #include "libc/sysv/errfuns.h"
 
 static void sigaltstack2bsd(struct sigaltstack_bsd *bsd,
@@ -76,6 +77,8 @@ static void sigaltstack2linux(struct sigaltstack *linux,
  * @param neu if non-null will install new signal alt stack
  * @param old if non-null will receive current signal alt stack
  * @return 0 on success, or -1 w/ errno
+ * @raise EFAULT if bad memory was supplied
+ * @raise ENOMEM if `neu->ss_size` is less than `MINSIGSTKSZ`
  */
 int sigaltstack(const struct sigaltstack *neu, struct sigaltstack *old) {
   int rc;
@@ -86,6 +89,8 @@ int sigaltstack(const struct sigaltstack *neu, struct sigaltstack *old) {
                    (neu && (__asan_check(neu, sizeof(*neu)).kind ||
                             __asan_check(neu->ss_sp, neu->ss_size).kind)))) {
     rc = efault();
+  } else if (neu && neu->ss_size < MINSIGSTKSZ) {
+    rc = enomem();
   } else if (IsLinux() || IsBsd()) {
     if (IsLinux()) {
       a = neu;