Move terminal-related selection logic into st.c

Move terminal-related selection logic into st.c

The front-end determines information about mouse clicks and motion, and
the terminal handles the actual selection start/extend/dirty logic by
row and column.

While we're in the neighborhood, we'll also rename getbuttoninfo() to
mousesel() which is, at least, less wrong.

Signed-off-by: Devin J. Pohly <djpohly@gmail.com>