Code review for magic-block patch. Remove separate header file pgmagic.h,

Code review for magic-block patch.  Remove separate header file pgmagic.h,
as this seems only likely to create headaches for module developers.  Put
the macro in the pre-existing fmgr.h file instead.  Avoid being too cute
about how many fields we can cram into a word, and avoid trying to fetch
from a library we've already unlinked.
Along the way, it occurred to me that the magic block really ought to be
'const' so it can be stored in the program text area.  Do the same for
the existing data blocks for PG_FUNCTION_INFO_V1 functions.