Use readline's 'history_max_entries' instead of the old 'max_input_history'.

Use readline's 'history_max_entries' instead of the old 'max_input_history'.

Ref: http://www.sourceware.org/ml/gdb-patches/2002-08/msg00486.html

We've long since imported a newer readline, no need to use the old
compatibility variable anymore.

Tested on x86_64 Fedora 17.

gdb/
2013-03-26  Pedro Alves  <palves@redhat.com>

	* top.c (gdb_rl_operate_and_get_next): Replace max_input_history
	use with history_max_entries use.  Remove FIXME note.