databases/pgbouncer: Add default option MANPAGES

Port depends on hs-pandoc to build its manpages, that adds a dependency
on lang/ghc, which takes hours to build.
MANPAGES option save lots of build time for users who build their own
ports without MANPAGES.

While here fix build with custom USER/GROUP not from UIDs/GIDs.

PR: 295239
Approved by: Dmitry Wagin <dmitry.wagin@ya.ru> (maintainer, timeout 2 weeks)
Sponsored by: UNIS Labs