Add support for a rescue (recovery) mode from kernel cmdline

This patch adds a rescue (revovery) mode to Finit.  Simply add `rescue`
to the kernel cmdline at boot and the following steps at bootstrap will
be skipped:

- Load of services/run/task/etc in /etc/finit.conf + /etc/finit.d/*.conf
- fsck of filesystems in /etc/fstab
- Remount of / to read-write
- Mount of all filesystems in /etc/fstab
- swapon
- A few plugin hooks:
  - HOOK_ROOTFS_UP
  - HOOK_MOUNT_ERROR
  - HOOK_BASEFS_UP
- `runparts DIR`
- /etc/rc.local

Instead of loading /etc/finit.conf and /etc/finit.d/*.conf Finit loads
the file /lib/finit/rescue.conf, which can be overridden by the operator
if needed.  If this file is removed (or lost), Finit falls back to start
a simple root shell, without any login.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

src/conf.c

@@ -78,6 +78,9 @@ void conf_parse_cmdline(void)
 		/* Catches finit_debug (deprecated), --debug, and debug */
 		if (strstr(tok, "debug"))
 			dbg = 1;
+
+		if (strstr(tok, "rescue") || strstr(tok, "recover"))
+			rescue = 1;
 	}
 	fclose(fp);
 
@@ -551,6 +554,19 @@ int conf_reload(void)
 	svc_mark_dynamic();
 	tty_mark();
 
+	if (rescue) {
+		int rc;
+		char line[80] = "tty [12345] @console noclear nologin";
+
+		/* If rescue.conf is missing, fall back to a root shell */
+		rc = parse_conf(RESCUE_CONF);
+		if (rc)
+			tty_register(line, global_rlimit, NULL);
+
+		print(rc, "Entering rescue mode");
+		goto done;
+	}
+
 	/* First, read /etc/finit.conf */
 	parse_conf(FINIT_CONF);
 
@@ -795,6 +811,10 @@ int conf_monitor(uev_ctx_t *ctx)
 {
 	int rc = 0;
 
+	/* Skip second run, when called from finit.c in rescue mode */
+	if (ctx && rescue)
+		return 0;
+
 	/*
 	 * If only one watcher fails, that's OK.  A user may have only
 	 * one of /etc/finit.conf or /etc/finit.d in use, and may also