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

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.