Add an option -Q (quick) to mandocdb(8)

Add an option -Q (quick) to mandocdb(8)
for accelerated generation of reduced-size databases.

Implement this by allowing the parsers to optionally
abort the parse sequence after the NAME section.

While here, garbage collect the unused void *arg attribute
of struct mparse and mparse_alloc().

This reduces the processing time of mandocdb(8) on /usr/share/man
by a factor of 2 and the database size by a factor of 4.
However, it still takes 5 times the time and 6 times the space
of makewhatis(8), so more work is clearly needed.