memcoll: clarify sizes versus lengths, document better, and tweak perf

memcoll: clarify sizes versus lengths, document better, and tweak perf

* lib/memcoll.c (strcoll_loop, memcoll0):
Improve quality of descriptive comments.  Name variables
consistently as to whether they are lengths (which do not include
terminating null) versus sizes (which do).
* lib/xmemcoll.c (xmemcoll0): Likewise.
* lib/memcoll.c (strcoll_loop): Tweak the way that the diff is
returned when s1size == 0; this is easier to compile and saves
about 17% of memcoll's code space on x86-64 with GCC 4.1.2.