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.

config.def.h

@@ -11,13 +11,14 @@ static int borderpx = 2;
 /*
  * What program is execed by st depends of these precedence rules:
  * 1: program passed with -e
- * 2: utmp option
+ * 2: scroll and/or utmp
  * 3: SHELL environment variable
  * 4: value of shell in /etc/passwd
  * 5: value of shell in config.h
  */
 static char *shell = "/bin/sh";
 char *utmp = NULL;
+char *scroll = NULL;
 char *stty_args = "stty raw pass8 nl -echo -iexten -cstopb 38400";
 
 /* identification sequence returned in DA and DECID */