Add /usr/sbin and /sbin to default root PATH

When dropbear is used in a very restricted environment (such as in a
initrd), the default user shell is often also very restricted
and doesn't take care of setting the PATH so the user ends up
with the PATH set by dropbear. Unfortunately, dropbear always
sets "/usr/bin:/bin" as default PATH even for the root user
which should have /usr/sbin and /sbin too.

For a concrete instance of this problem, see the "Remote Unlocking"
section in this tutorial: https://paxswill.com/blog/2013/11/04/encrypted-raspberry-pi/

It speaks of a bug in the initramfs script because it's written "blkid"
instead of "/sbin/blkid"... this is just because the scripts from the
initramfs do not expect to have a PATH without the sbin directories and
because dropbear is not setting the PATH appropriately for the root user.

I'm thus suggesting to use the attached patch to fix this misbehaviour (I
did not test it, but it's easy enough). It might seem anecdotic but
multiple Kali users have been bitten by this.

From https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=903403

default_options.h

@@ -339,5 +339,6 @@ be overridden at runtime with -I. 0 disables idle timeouts */
 
 /* The default path. This will often get replaced by the shell */
 #define DEFAULT_PATH "/usr/bin:/bin"
+#define DEFAULT_ROOT_PATH "/usr/sbin:/usr/bin:/sbin:/bin"
 
 #endif /* DROPBEAR_DEFAULT_OPTIONS_H_ */

svr-chansession.c

@@ -1012,7 +1012,11 @@ static void execchild(const void *user_data) {
 	addnewvar("LOGNAME", ses.authstate.pw_name);
 	addnewvar("HOME", ses.authstate.pw_dir);
 	addnewvar("SHELL", get_user_shell());
-	addnewvar("PATH", DEFAULT_PATH);
+	if (getuid() == 0) {
+		addnewvar("PATH", DEFAULT_ROOT_PATH);
+	} else {
+		addnewvar("PATH", DEFAULT_PATH);
+	}
 	if (cp != NULL) {
 		addnewvar("LANG", cp);
 		m_free(cp);