constify to_open

This makes target_ops::to_open take a const string and then fixes the
fallout.

There were a few of these I could not build.  However I eyeballed it
and in any case the fixes should generally be trivial.

This is based on the patch to fix up the target debugging for to_open,
because that changes gdb to not directly install to_open as the target
command

2014-07-30  Tom Tromey  <tromey@redhat.com>

	* bsd-kvm.c (bsd_kvm_open): Constify.
	* corelow.c (core_open): Constify.
	* ctf.c (ctf_open): Constify.
	* dbug-rom.c (dbug_open): Constify.
	* exec.c (exec_open): Constify.
	* m32r-rom.c (m32r_open, mon2000_open): Constify.
	* microblaze-rom.c (picobug_open): Constify.
	* nto-procfs.c (procfs_open_1, procfs_open, procfs_native_open):
	Constify.
	* ppcbug-rom.c (ppcbug_open0, ppcbug_open1): Constify.
	* record-btrace.c (record_btrace_open): Constify.
	* record-full.c (record_full_core_open_1, record_full_open_1)
	(record_full_open): Constify.
	* remote-m32r-sdi.c (m32r_open): Constify.
	* remote-mips.c (common_open, mips_open, pmon_open, ddb_open)
	(rockhopper_open, lsi_open): Constify.
	* remote-sim.c (gdbsim_open): Constify.
	* remote.c (remote_open, extended_remote_open, remote_open_1):
	Constify.
	* target.h (struct target_ops) <to_open>: Make "arg" const.
	* tracefile-tfile.c (tfile_open): Constify.

gdb/target.h

@@ -405,7 +405,7 @@ struct target_ops
        command, and (if successful) pushes a new target onto the
        stack.  Targets should supply this routine, if only to provide
        an error message.  */
-    void (*to_open) (char *, int);
+    void (*to_open) (const char *, int);
     /* Old targets with a static target vector provide "to_close".
        New re-entrant targets provide "to_xclose" and that is expected
        to xfree everything (including the "struct target_ops").  */