Fix warnings with lib/libpmc

Fix warnings with lib/libpmc

* Use `MIN` instead of similar hand rolled macro.
* Sort headers.
* Use `errno.h` instead of `sys/errno.h`.
* Wrap the argument to sizeof in parentheses for clarity.
* Remove `__BSD_VISIBLE` and `_XOPEN_SOURCE` #defines to mute warnings about
  incompatible snprintf definitions.

This fixes a number of warnings I've been seeing lately in my builds.

Sort makefile variables per style.Makefile(9) (`CFLAGS`/`CWARNFLAG.gcc`) and
bump `WARNS` to 3.

MFC after:	2 weeks
Reviewed by:	jtl
Approved by:	jtl (mentor)
MFC after:	1 month
Differential Revision:	https://reviews.freebsd.org/D19851