man: Exit cleanly on SIGPIPE.

The first attempt at addressing this simply suppressed SIGPIPE, which
resulted in mandoc printing out error messages instead. This was then
reverted, but the pipefail was (correctly) left in, so man still
returned a nonzero exit code if you quit a page before the end.

PR: 223516, 279542
Fixes: 14a5c1068d37, a85d870007e7
MFC after: 1 week
Reviewed by: ziaee, kevans
Differential Revision: https://reviews.freebsd.org/D50302