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.

1 files changed, 1 insertions, 1 deletions

diff --git a/gdb/top.c b/gdb/top.c
index 0a4d36c..4a0fedb 100644
--- a/gdb/top.c
+++ b/gdb/top.c
@@ -1846,7 +1846,7 @@ set_verbose (const char *args, int from_tty, struct cmd_list_element *c)
 void
 init_history (void)
 {
-  char *tmpenv;
+  const char *tmpenv;
 
   tmpenv = getenv ("GDBHISTSIZE");
   if (tmpenv)