Convert Commands to use the union pointer_long_t too.
As with MuttVars, Commands was using an unsigned long to hold pointers
as well as enums. Convert to use the same union type of MuttVars.
Adjust command functions data parameter type to the union. Since
these are used outside init.c, relocate the union definition to
mutt.h.
Although the number of functions affected was long, most of them did
not need much adjustment. Many of them made no use of the parameter.
Those that did were easily cast into an added "data" variable at the
top.