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/intrin/tls.greg.c

@@ -92,9 +92,7 @@ privileged void __install_tls(char tib[64]) {
   assert(!__tls_enabled);
   assert(*(int *)(tib + 0x38) != -1);
   if (IsWindows()) {
-    if (!__tls_index) {
-      __tls_index = TlsAlloc();
-    }
+    __tls_index = TlsAlloc();
     asm("mov\t%1,%%gs:%0" : "=m"(*((long *)0x1480 + __tls_index)) : "r"(tib));
   } else if (IsFreebsd()) {
     asm volatile("syscall"