Rename VACOPT_NOWAIT to VACOPT_SKIP_LOCKED

Rename VACOPT_NOWAIT to VACOPT_SKIP_LOCKED

When it comes to SELECT ... FOR or LOCK, NOWAIT means to not wait for
something to happen, and issue an error.  SKIP LOCKED means to not wait
for something to happen but to move on without issuing an error.  The
internal option of autovacuum and autoanalyze mentioned above, used only
when wraparound is not involved was named NOWAIT, but behaves like SKIP
LOCKED which is confusing.

Author: Nathan Bossart
Discussion: https://postgr.es/m/20180307050345.GA3095@paquier.xyz