strncmp->memcmp when we know the shorter length

When the caller knows the smaller string length, memcmp and strncmp are
functionally equivalent.  Since memcmp need not watch each byte for a NULL
terminator, it often compares a CPU word at a time for better performance.  The
attached patch changes use of strncmp to memcmp where we have the length of the
shorter string.  I was most interested in the varlena.c instances, but I tried
to find all applicable call sites.  To benchmark it, I used the attached
"bench-texteq.sql".  This patch improved my 5-run average timing of the SELECT
from 65.8s to 56.9s, a 13% improvement.  I can't think of a case where the
change should be pessimal.

Thanks,
nm