proc_uuid.cc: Clean up code

proc_uuid.cc: Clean up code

Use UUID_STRING_SIZE consistently.

Don't bother nul-terminating UUID read from /proc/sys/kernel/random/uuid
before passing it to uuid_parse() since our uuid_parse doesn't care if
there's a terminating nul or not.

Make uuid_unparse_lower() nul-terminate its result.  The glass code
doesn't currently need this, but it's a potentially surprising
difference compared to the util-linux-ng implementation we're emulating.