MFC r363630:

MFC r363630:

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
Sponsored by:   iXsystems, Inc.
Differential Revision:  https://reviews.freebsd.org/D25679