gdb: deprecated filename_completer and associated functions

gdb: deprecated filename_completer and associated functions

Following on from the previous commit, this commit marks the old
unquoted filename completion related functions as deprecated.

The aim of doing this is to make it more obvious to someone adding a
new command that they should not be using the older unquoted style
filename argument handling.

I split this change from the previous to make for an easier review.
This commit touches more files, but is _just_ function renaming.
Check out gdb/completer.{c,h} for what has been renamed.  All the
other files have just been updated to use the new names.

There should be no user visible changes after this commit.