Add support for scroll(1)

Scroll is a program that stores all the lines of its child and be used in st as
a way of implementing scrollback.

This solution is much better than implementing the scrollback in st itself
because having a different program allows to use it in any other program
without doing modifications to those programs.

st.h

@@ -113,6 +113,7 @@ char *xstrdup(char *);
 
 /* config.h globals */
 extern char *utmp;
+extern char *scroll;
 extern char *stty_args;
 extern char *vtiden;
 extern wchar_t *worddelimiters;