Change get_inferior_args to return const char *

Change get_inferior_args to return const char *

I noticed that get_inferior_args should return const char *, because
it is just returning a reference to something owned by the inferior.

I'm checking this in.

gdb/ChangeLog
2018-09-15  Tom Tromey  <tom@tromey.com>

	* infcmd.c (get_inferior_args): Return const char *.
	* inferior.h (get_inferior_args): Return type now const.
	* linux-tdep.c (linux_fill_prpsinfo): Update.
	* procfs.c (procfs_target::make_corefile_notes): Update.