Rearrange fmgr.c and relcache so that it's possible to keep FmgrInfo

lookup info in the relcache for index access method support functions.
This makes a huge difference for dynamically loaded support functions,
and should save a few cycles even for built-in ones.  Also tweak dfmgr.c
so that load_external_function is called only once, not twice, when
doing fmgr_info for a dynamically loaded function.  All per performance
gripe from Teodor Sigaev, 5-Oct-01.