Convert default_collect_symbol_completion_matches_break_on

This converts default_collect_symbol_completion_matches_break_on to
the callback approach, merging the search loop and the call to
expand_symtabs_matching.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=16994
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=16998
Acked-By: Simon Marchi <simon.marchi@efficios.com>