Improve error message when dtach can't find a pty.

Also add some documentation mentioning the devices that dtach usually
needs access to on Linux.

Bug reported by David Bremner at https://bugs.debian.org/804944

README

@@ -31,6 +31,10 @@ be easily portable to other variants of Unix. It currently assumes that
 the host system uses POSIX termios, and has a working forkpty function
 available.
 
+dtach may need access to various devices in the filesystem depending on what
+forkpty does. For example, dtach on Linux usually needs access to /dev/ptmx
+and /dev/pts.
+
 2. QUICK START
 
 Compiling dtach should be simple, as it uses autoconf:

master.c

@@ -455,6 +455,9 @@ master_process(int s, char **argv, int waitattach, int statusfd)
 	{
 		if (statusfd != -1)
 			dup2(statusfd, 1);
+		if (errno == ENOENT)
+			printf("%s: Could not find a pty.\n", progname);
+		else
 			printf("%s: init_pty: %s\n", progname, strerror(errno));
 		exit(1);
 	}