FScreenGetScrRect: Remove m == NULL check (GCC fix)

Because FScreenGetScrRect() initialises values passed in for coordinates,
don't test to see if the given monitor is NULL (which would have
resulted in the premature exit of this function, potentially leaving the
values unassigned).

We always guarantee finding a given monitor, and hence can set these values
correctly.  The original check was bogus, and caused GCC to rightly complain
about potential uninitialised values (-Werror=maybe-uninitialized).
Rename __execute_function -> __execute_command_line

By the time __execute_function() is called, the function/command involved
has already been identified.  Instead, call this __execute_command_line() to
more accurately reflect its job.
Parser notes:  update for __execute_command_line

Change the parsing notes to reflect name change of internal function.
Make note of the fact that __execute_function() is named as such in fvwm, to
avoid unncessary confusion.
!!! Start work on replacing the parsing in __execute_command_line ...

... with hook functions that are implemented elsewhere.  This aims
at allowing to writing and testing multiple parsers in parallel
and at moving all parsing and expansion stuff to a different
place.
!add missing files
Split off cmdparser.[ch] from cmdparser.[ch].
!!!more work on parser replacement.
work
add debug output and fix positional arguments
add debug output and fix command line prefixes
Convert special logic in __execute_command_line for functions ...

... that may move a window to a separate function flag.
Move find_builtin_function() to functable.[ch].
work
whitespace cleanup
Moved some functions to functable.[ch] and functable_complex.[ch].

This separation is needed for the parsing hooks.  It was very difficult to get
the #icludes right.  The DesktopsInfo structure was moved to libs/FScreen.h
so that the library does not depend on core header files anymore.
missing files
!!!fix
Move WindowConditionMask from mvwm.h to conditional.h.
!!!work
Finish 1st draft of hook implementation (__execute_command_line)