Constify some commands in exec.c, plus symbol_file_command

Constify some commands in exec.c, plus symbol_file_command

Note that this commit also changes deprecated_file_changed_hook -- not
used in the tree, but Insight will require a (presumably minor)
change.

gdb/ChangeLog
2017-09-27  Tom Tromey  <tom@tromey.com>

	* symfile.c (symbol_file_command): Constify.
	* gdbcore.h (deprecated_file_changed_hook): Constify.
	* exec.c (deprecated_file_changed_hook, exec_file_command)
	(file_command): Constify.
	* defs.h (symbol_file_command): Constify.