emacs: Don't install ctags and its manpage

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. Also, remove the conflict with ctags and add
a caveat to the formula about this whole issue.

Fixes #16826.

Signed-off-by: Misty De Meo <mistydemeo@gmail.com>
ctags: Remove conflict with emacs and add caveat

Closes #16841.

Signed-off-by: Misty De Meo <mistydemeo@gmail.com>