libpmc: Use known pmc_cpuid buffer size

[freqlabs]

Summary

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.

Sorry for the extra review noise mav, this ended up changing a bit after taking a final look at the original patch :)

[freqlabs]

Going to commit this tonight since there has been no objection

[mav]

With kernel part I am definitely agree, while about first part of user space just won't object. While second part of user-space just had no sense before, the first one at least had some sense, even though not willing much.