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/calls/clock_gettime.internal.h

@@ -9,6 +9,7 @@ typedef int clock_gettime_f(int, struct timespec *);
 extern clock_gettime_f *__clock_gettime;
 clock_gettime_f *__clock_gettime_get(bool *) hidden;
 int __clock_gettime_init(int, struct timespec *) hidden;
+int sys_clock_gettime_mono(struct timespec *) hidden;
 
 COSMOPOLITAN_C_END_
 #endif /* !(__ASSEMBLER__ + __LINKER__ + 0) */