Constify target_ops::follow_exec

Constify target_ops::follow_exec

I noticed that target_ops::follow_exec took a "char *" parameter,
where "const char *" would be more correct.  This patch changes this
(and related functions) to be constified.

Tested by rebuilding.

gdb/ChangeLog
2019-05-22  Tom Tromey  <tromey@adacore.com>

	* target.c (target_follow_exec): Constify parameter.
	* target-delegates.c: Rebuild.
	* remote.c (remote_target::follow_exec): Constify parameter.
	* infrun.c (follow_exec): Constify parameter.
	* target.h (struct target_ops) <follow_exec>: Constify parameter.
	(target_follow_exec): Likewise.