pmc: Provide full path to modules from kernel linker

This unifies the user object and kernel module paths in libpmcstat,
allows modules loaded from non-standard locations (e.g. from a user's
home directory when testing) to be found and, since buffer is what all
the warnings here use (they were never updated when buffer_modules were
added to pick based on where the file was found) has the side-effect of
ensuring the messages are correct.

This includes obsoleting the now-superfluous -k option in pmcstat.

This change breaks the hwpmc ABI and will be followed by a bump to the
pmc major version.

Reviewed by: jhb, jkoshy, mhorne
Differential Revision: https://reviews.freebsd.org/D40048