FIX: Clean up matching and parsing

Rename parse_field() to match_field() and parse_field_promisc() to
parse_field() to use more typical parser terminology in the FIX code.

Saves me from looking up what parse_field_promisc() does every time I
read the code...

Signed-off-by: Pekka Enberg <penberg@iki.fi>
tools/fix: Fix SIGSEGV in fix_server.c

Signed-off-by: Pekka Enberg <penberg@iki.fi>