libpmc: Use known pmc_cpuid buffer size

Use the existing PMC_CPUID_LEN to size pmc_cpuid in the kernel and various
buffers for reading it in libpmc. This avoids some extra syscalls and
malloc/frees.

While in here, use strlcpy to copy a user-provided cpuid string instead of
memcpy, to make sure we terminate the buffer.

Reviewed by: mav
MFC after: 1 week
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D25679