Emacs versus ctags

In addition to its etags executable, emacs now installs a ctags executable.
This stomps over exuberant-ctags, and makes it difficult to have emacs and
vim installed and use tags for both.

This follows what MacPorts does and removes bin/ctags and its manpage from
the build before linking. An option is provided for users who want to keep
`ctags` from emacs. The conflict between ctags and emacs is removed from
both formulas, but each formula gets a caveat to let users understand their
choices.

This should fix #16826, #11089 and #8859.