bug#15641: 24.3; [PATCH] Add find-definition for M-.

[Barry OReilly]

This would be very nice improvement. When I'm editing C or C++, I
usually try semantic-ia-fast-jump first, which can fail to find the
tag for a variety of reasions. I use etags as my fallback, sometimes
with a few iterations with prefix-arg because of etags false results.

I've missed the fact that I can't pop positions with Semantic. In
practice I end up using Evil's C-o command for the same buffer or
buffer-menu if Semantic took me to a different buffer.

Currently semantic-ia-fast-jump doesn't prompt for the tag to find, as
etags' find-tag does. This is nicer in Semantic's case, so I think the
minibuffer prompting should be specific to etags and not a part of the
generic interface.

Currently find-tag takes TAGNAME while semantic-ia-fast-jump takes PT,
so their interfaces need to be consolidated. PT is more basic since
the TAGNAME etags offers as default is derived from what's at point.