[dev] [st] alternate screen switching

Hi,

attached is a patch, that adds support for the deprecated '\E[?47h' and
'\E[?47l' control sequences, which are used when $TERM == "xterm-color".

I've also added {rm,sm}cup entries to st.info, so that switching between
normal and alternate screen now works when $TERM == "st[-256color]".

I would also like to look into the scrollback buffer goal. How do you
want to achieve an unlimited scrollback buffer? Writing every line to a
file and printing its content instead of the current buffer when
scrolling back? What are the cons to just having a fixed amount of lines
(simply a bigger Line *line), memmoving them when input arrives and
normally displaying only the row-1 last ones?

Bert