gdb: introduce iteration_status enum, use it for search callbacks

There are a bunch of iteration functions that take a callback returning
true or false to indicate whether to continue or stop iterating.  These
functions then return the same value, indicate whether the iteration was
done until the end of interrupted.  I think this is confusing and
error-prone, as I never know which value means what.  It is especially
confusing when two opposite conventions collide, such as in
objfile::map_symtabs_matching_filename.

I propose to make that more obvious by introducing a new
iteration_status enum with self-documenting values.

I started to change the callback type
compunit_symtab_iteration_callback, taken by
quick_symbol_functions::search, and then followed that path to update a
bunch of other functions.

I chose the name to be kind of generic, so that it can be used for other
similar iteration patterns.  I also put it in gdbsupport, in case we
want to use it in gdbserver too.

Change-Id: I55d84d0c1af8ac0b82cc9f49ccf0d6b60e1769e0
Approved-By: Andrew Burgess <aburgess@redhat.com>