Rename "mem" related commmands

Rename "mem" related commmands

Rename "mem" related commands, so that their naming is consistent with
the <command-name>_command pattern of naming functions that implement
commands.

gdb/ChangeLog:

	* memattr.c (mem_info_command): Rename to ...
	(info_mem_command): ... this.
	(mem_enable_command): Rename to ...
	(enable_mem_command): ... this.
	(mem_disable_command): Rename to ...
	(disable_mem_command): ... this.
	(mem_delete_command): Rename to ...
	(delete_mem_command): ... this.
	(_initialize_mem): Adjust function names.