makefs: Fix warnings and reset WARNS to the default

Leave -Wcast-align disabled, at least for now, since there are numerous
instances of that warning in places where buffer pointers are cast to
pointers to various filesystem structures. Fixing this properly would
be too much work for too little gain.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit cc1a53bc1aea0675d64e9547cdca241612906592)