Infrastructure for I/O of composite types: arrange for the I/O routines

Infrastructure for I/O of composite types: arrange for the I/O routines
of a composite type to get that type's OID as their second parameter,
in place of typelem which is useless.  The actual changes are mostly
centralized in getTypeInputInfo and siblings, but I had to fix a few
places that were fetching pg_type.typelem for themselves instead of
using the lsyscache.c routines.  Also, I renamed all the related variables
from 'typelem' to 'typioparam' to discourage people from assuming that
they necessarily contain array element types.