Add "const" to a few locals in gdb

Add "const" to a few locals in gdb

I noticed that some code in gdb was doing:

    char *mumble = getenv (...)

However, using "const char *" here would be clearer.
This patch fixes the instances I could readily build.

Tested by rebuilding.

gdb/ChangeLog
2018-09-24  Tom Tromey  <tom@tromey.com>

	* common/pathstuff.c (get_standard_cache_dir): Make
	"xdg_cache_home" and "home" const.
	* top.c (init_history): Make "tmpenv" const.
	* main.c (get_init_files): Make "homedir" const.