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