Add new command "maint print core-file-backed-mappings"

Add new command "maint print core-file-backed-mappings"

I wrote a read_core_file_mappings method for FreeBSD and then registered
this gdbarch method.  I saw some strange behavior while testing it and
wanted a way to make sure that mappings were being correctly loaded
into corelow.c, so I wrote the new command which is the topic of this
commit.  I think it might be occasionally useful for debugging strange
corefile behavior.

With regard to FreeBSD, my work isn't ready yet.  Unlike Linux,
FreeBSD puts all mappings into its core file note.  And, unlike Linux,
it doesn't dump load segments which occupy no space in the file.  So
my (perhaps naive) implementation of a FreeBSD read_core_file_mappings
didn't work all that well:  I saw more failures in the corefile2.exp
tests than without it.  I think it should be possible to make FreeBSD
work as well as Linux, but it will require doing something with all of
the mappings, not just the file based mappings that I was considering.

In the v4 series, Pedro asked the following:

    I don't understand what this command provides that "info proc
    mappings" doesn't?  Can you give an example of when you'd use this
    command over "info proc mappings" ?

On Linux, "info proc mappings" and "maint print core-file-backed-mappings"
will produce similar, possibly identical, output.  This need not be
the case for other OSes.  E.g. on FreeBSD, had I finished the
implementation, the output from these commands would have been very
different.  The FreeBSD "info proc mappings" command would show
additional (non-file-backed) mappings in addition to at least one
additional field (memory permissions) for each mapping.

As noted earlier, I was seeing some unexpected behavior while working
on the FreeBSD implementation and wanted to be certain that the
mappings were being correctly loaded by corelow.c.  "info proc
mappings" prints the core file mappings, but doesn't tell us anything
about whether they've been loaded by corelow.c This new maintenance
command directly interrogates the data structures and prints the
values found there.

gdb/ChangeLog:

	* corelow.c (gdbcmd.h): Include.
	(core_target::info_proc_mappings): New method.
	(get_current_core_target): New function.
	(maintenance_print_core_file_backed_mappings): New function.
	(_initialize_corelow): Add core-file-backed-mappings to
	"maint print" commands.