Stop querying for fonts as soon as a match is found

Stop querying for fonts as soon as a match is found

Scanning through fonts can be very slow, especially with the 'x' font
backend, and the result is almost always not used.  Stop looking for a
font as soon as one is found rather than scanning all the backends.

* src/font.c (font_list_entities): Stop scanning through the font
backends as soon as we find a match unless
'query-all-font-backends is set (Bug#43177).
(syms_of_font): New variable 'query-all-font-backends', default
false.