Add more apis to redbean unix module

- Document unix.fcntl()
- Add POSIX Advisory Locks
- Add mask parameter to unix.poll()
- Add lowest parameter to unix.dup()

libc/calls/struct/flock.h

@@ -7,7 +7,7 @@ struct flock {      /* cosmopolitan abi */
   int16_t l_type;   /* F_RDLCK, F_WRLCK, F_UNLCK */
   int16_t l_whence; /* SEEK_SET, SEEK_CUR, SEEK_END */
   int64_t l_start;  /* starting offset */
-  int64_t l_len;    /* 0 means until end of file */
+  int64_t l_len;    /* no. bytes (0 means to end of file) */
   int32_t l_pid;    /* lock owner */
   int32_t l_sysid;  /* remote system id or zero for local */
 };