Do not sync to disk after each individual manual page (duh!),

Do not sync to disk after each individual manual page (duh!),
only sync to disk one single time when all data is ready.

Rebuild times for /usr/share/man/mandoc.db shrink on my notebook:
In standard mode from 45 seconds to 11 seconds   (75% reduction)
In -Q mode       from 25 seconds to 3.1 seconds  (87% reduction)
For comparison: makewhatis(8):      4.2 seconds

That is, in -Q mode, we are now *faster* than the existing makewhatis(8),
and careful profiling shows there is still a lot of room for improval.