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 */

st.1

@@ -170,7 +170,8 @@ See the LICENSE file for the terms of redistribution.
 .SH SEE ALSO
 .BR tabbed (1),
 .BR utmp (1),
-.BR stty (1)
+.BR stty (1),
+.BR scroll (1)
 .SH BUGS
 See the TODO file in the distribution.
 

st.c

@@ -664,7 +664,7 @@ die(const char *errstr, ...)
 void
 execsh(char *cmd, char **args)
 {
-	char *sh, *prog;
+	char *sh, *prog, *arg;
 	const struct passwd *pw;
 
 	errno = 0;
@@ -678,13 +678,17 @@ execsh(char *cmd, char **args)
 	if ((sh = getenv("SHELL")) == NULL)
 		sh = (pw->pw_shell[0]) ? pw->pw_shell : cmd;
 
-	if (args)
+	if (args) {
 		prog = args[0];
-	else if (utmp)
-		prog = utmp;
-	else
+		arg = NULL;
+	} else if (scroll || utmp) {
+		prog = scroll ? scroll : utmp;
+		arg = scroll ? utmp : NULL;
+	} else {
 		prog = sh;
-	DEFAULT(args, ((char *[]) {prog, NULL}));
+		arg = NULL;
+	}
+	DEFAULT(args, ((char *[]) {prog, arg, NULL}));
 
 	unsetenv("COLUMNS");
 	unsetenv("LINES");

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;