hwpmc: don't use deprecated copystr(9)

It is just wrapper around strlcpy(), but results in more complicated
code. Clean this up to use strlcpy() or snprintf() as appropriate.

Reviewed by: jkoshy
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39910

(cherry picked from commit 31610e34b7e39d573d927ea98346c88f23cabdab)