MFC r317179:

Clean up WITH*_PROFILE

- Clarify that profiled libraries are usable with gprof(8).
- Add WITH_PROFILE to complement WITHOUT_PROFILE and placate makeman.

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Revert to conservative defaults. WITH_PROFILE is a default now.

NO_PROFILE overrides in Makefile.inc1 and bsd.own.m are left intact
so that the reversal to the WITHOUT_PROFILE will be easier in future.

Turn off profiled libs build by default.
Can be enabled back using WITH_PROFILE=yes in /etc/src.conf

Prepare to autogenerate the src.conf(5) manpage.