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/sysv/consts/timer.h

@@ -0,0 +1,6 @@
+#ifndef COSMOPOLITAN_LIBC_SYSV_CONSTS_TIMER_H_
+#define COSMOPOLITAN_LIBC_SYSV_CONSTS_TIMER_H_
+
+#define TIMER_ABSTIME 1
+
+#endif /* COSMOPOLITAN_LIBC_SYSV_CONSTS_TIMER_H_ */