replace the numbers with UID_NOBODY, GID_NOGROUP in mountd.c

[rmacklem]

Summary

Since UID_NOBODY, GID_NOGROUP were not exposed to userland,
mountd.c has the numbers hard coded in it.
This patch exposes UID_NOBODY, GID_NOGROUP to userland
and mountd.c is changed to use them.

Suggested by kib@.

Test Plan

It compiles fine and find/grep did not locate any uses
of the UID_NOBODY, GID_NOGROUP constants in
the userland part of /usr/src.