Allow overriding shells for compat getusershell()

compat.c

@@ -231,8 +231,7 @@ void setusershell() {
 }
 
 static char **initshells() {
-	/* don't touch this list. */
-	static const char *okshells[] = { "/bin/sh", "/bin/csh", NULL };
+	static const char *okshells[] = { COMPAT_USER_SHELLS, NULL };
 	register char **sp, *cp;
 	register FILE *fp;
 	struct stat statb;

sysoptions.h

@@ -79,6 +79,15 @@
 
 #define _PATH_CP "/bin/cp"
 
+/* Default contents of /etc/shells if system getusershell() doesn't exist.
+ * Paths taken from getusershell(3) manpage. These can be customised
+ * on other platforms. One the commandline for CFLAGS it would look like eg
+  -DCOMPAT_USER_SHELLS='"/bin/sh","/apps/bin/sh","/data/bin/zsh"
+ */
+#ifndef COMPAT_USER_SHELLS
+#define COMPAT_USER_SHELLS "/bin/sh","/bin/csh"
+#endif
+
 #define DROPBEAR_ESCAPE_CHAR '~'
 
 /* success/failure defines */