Improve system calls

- Wrap clock_getres()
- Wrap sched_setscheduler()
- Make sleep() api conformant
- Polyfill sleep() using select()
- Improve clock_gettime() polyfill
- Make nanosleep() POSIX conformant
- Slightly improve some DNS functions
- Further strengthen pledge() sandboxing
- Improve rounding of timeval / timespec
- Allow layering of pledge() calls on Linux
- Polyfill sched_yield() using select() on XNU
- Delete more system constants we probably don't need

libc/dns/herror.c

@@ -25,12 +25,12 @@
 │ OTHER DEALINGS IN THE SOFTWARE.                                              │
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/dns/ent.h"
+#include "libc/stdio/stdio.h"
 
-int h_errno;
-
+/**
+ * Prints `h_errno` description to stderr.
+ * @see perror()
+ */
 void herror(const char *s) {
-}
-
-const char *hstrerror(int err) {
-  return "unknown";
+  fprintf(stderr, "%s%s%s\n", s ? s : "", s ? ": " : "", hstrerror(h_errno));
 }