man perlrules doesn't always output "No manual entry for perlrules"

especially when the locale is non-English.

Hopefully all Linux dists are producing the same message, and force the
language to "C" so we get English messages.
(cherry picked from commit 08a004e4d58edacd87d6e8a443a269a6658faf4c)
On Linux LC_ALL overrides LC_MESSAGES, so tweak perl5db.t accordingly.

Without this, the test fails when LC_ALL is set to a non-English locale for
which man has been localised.
(cherry picked from commit f81b1f8b75b4a650b1375b4226116b035f86cb09)