Implement 'set dump-excluded-mappings' command

Implement 'set dump-excluded-mappings' command

Commit df8411da087dc05481926f4c4a82deabc5bc3859 implemented support for
checking /proc/PID/coredump_filter, and also changed gcore behavior to
unconditionally honor the VM_DONTDUMP flag, preventing sections marked
as such for being dumped into the core file.

This patch implements the 'set dump-excluded-mappings' command for
instructing gdb to ignore the VM_DONTDUMP flag. Combined with 'set
use-coredump-filter', this allows the user to restore the old behavior,
dumping all sections (except the ones marked as IO) unconditionally.

gdb/Changelog:
2017-11-29  Sergio Lopez  <slp@redhat.com>

	* linux-tdep.c (dump_excluded_mappings): New variable.
	(dump_mapping_p): Use dump_excluded_mappings variable.
	(_initialize_linux_tdep): New command 'set dump_excluded_mappings'.