Tweak a few of the most heavily used function call points to zero out

Tweak a few of the most heavily used function call points to zero out
just the significant fields of FunctionCallInfoData, rather than MemSet'ing
the whole struct to zero.  Unused positions in the arg[] array will
thereby contain garbage rather than zeroes.  This buys back some of the
performance hit from increasing FUNC_MAX_ARGS.  Also tweak tuplesort.c
code for more speed by marking some routines 'inline'.  All together
these changes speed up simple sorts, like count(distinct int4column),
by about 25% on a P4 running RH Linux 7.2.