The contents of command.c, creatinh.c, define.c, remove.c and rename.c

The contents of command.c, creatinh.c, define.c, remove.c and rename.c
have been divided according to the type of object manipulated - so ALTER
TABLE code is in tablecmds.c, aggregate commands in aggregatecmds.c and
so on.

A few common support routines remain in define.c (prototypes in
src/include/commands/defrem.h).

No code has been changed except for includes to reflect the new files.
The prototypes for aggregatecmds.c, functioncmds.c, operatorcmds.c,
and typecmds.c remain in src/include/commands/defrem.h.

From John Gray <jgray@azuli.co.uk>