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

tool/net/help.txt

@@ -1095,7 +1095,7 @@ FUNCTIONS
           operating system to choose a port, which may be revealed later on
           by GetServerAddr or the -z flag to stdout.
 
-  ProgramMaxPayloadSize(int64)
+  ProgramMaxPayloadSize(int)
           Sets the maximum HTTP message payload size in bytes. The
           default is very conservatively set to 65536 so this is
           something many people will want to increase. This limit is