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/setpriority.c

@@ -20,22 +20,38 @@
 #include "libc/calls/syscall-nt.internal.h"
 #include "libc/calls/syscall-sysv.internal.h"
 #include "libc/dce.h"
+#include "libc/intrin/describeflags.internal.h"
+#include "libc/intrin/strace.internal.h"
 
 /**
  * Sets nice value of thing.
  *
- * @param which can be PRIO_PROCESS, PRIO_PGRP, PRIO_USER
+ * On Windows, there's only six priority classes. We define them as -16
+ * (realtime), -10 (high), -5 (above), 0 (normal), 5 (below), 15 (idle)
+ * which are the only values that'll roundtrip getpriority/setpriority.
+ *
+ * @param which can be one of:
+ *     - `PRIO_PROCESS` is supported universally
+ *     - `PRIO_PGRP` is supported on unix
+ *     - `PRIO_USER` is supported on unix
  * @param who is the pid, pgid, or uid, 0 meaning current
  * @param value ∈ [-NZERO,NZERO) which is clamped automatically
- * @return 0 on success or -1 w/ errno
- * @error EACCES if lower that RLIMIT_NICE
- * @error EACCES on Linux without CAP_SYS_NICE
- * @see getpriority(), nice()
+ * @return 0 on success, or -1 w/ errno
+ * @raise EINVAL if `which` was invalid or unsupported
+ * @error EACCES if `value` lower that `RLIMIT_NICE`
+ * @error EACCES on Linux without `CAP_SYS_NICE`
+ * @raise EPERM if access to process was denied
+ * @raise ESRCH if the process didn't exist
+ * @see getpriority()
  */
 int setpriority(int which, unsigned who, int value) {
+  int rc;
   if (!IsWindows()) {
-    return sys_setpriority(which, who, value);
+    rc = sys_setpriority(which, who, value);
   } else {
-    return sys_getsetpriority_nt(which, who, value, sys_setpriority_nt);
+    rc = sys_setpriority_nt(which, who, value);
   }
+  STRACE("setpriority(%s, %u, %d) → %d% m", DescribeWhichPrio(which), who,
+         value, rc);
+  return rc;
 }