Make remote.c ptid constants "const"

Make remote.c ptid constants "const"

This changes magic_null_ptid, not_sent_ptid, and any_thread_ptid to be
"const".  This is a minor improvement that makes it so these can't be
accidentally modified.

Tested by rebuilding.  I'm checking this in.

gdb/ChangeLog
2019-03-12  Tom Tromey  <tromey@adacore.com>

	* remote.c (magic_null_ptid, not_sent_ptid, any_thread_ptid): Now
	const.  Add initializers.
	(_initialize_remote): Don't initialize ptid globals.