Refactor string_to_event_location for legacy linespec support.

This patch refactors string_to_event_location, breaking it into two
separate functions:

1) string_to_event_location_basic
A "basic" string parser that implements support for "legacy" linespecs
(linespec, address, and probe locations).  This function is intended to
be used by any UI wishing/needing to support this legacy behavior.

2) string_to_event_location
This is now intended as a CLI-only function which adds explicit location
parsing in a CLI-appropriate manner (in the form of traditional option/value
pairs).

Together these patches serve to simplify string-to-event location parsing
for all existing non-CLI interfaces (MI, guile, and python).

gdb/ChangeLog

	* location.c (string_to_explicit_location): Note that "-p" is
	reserved for probe locations and return NULL for any input
	that starts with that.
	(string_to_event_location): Move "legacy" linespec code to ...
	(string_to_event_location_basic): ... here.
	* location.h (string_to_event_location): Update comment.
	(string_to_event_location_basic): New function.