mount: recognize "dirsync" (closes bug 835)

mount: sanitize environ if called by non-root
*: adjust for slightly different sanitize routine

include/libbb.h

@@ -684,8 +684,8 @@ enum {
 #endif
 void bb_daemonize_or_rexec(int flags, char **argv);
 void bb_sanitize_stdio(void);
-/* Clear dangerous stuff, set PATH */
-void sanitize_env_for_suid(void);
+/* Clear dangerous stuff, set PATH. Return 1 if was run by different user. */
+int sanitize_env_if_suid(void);
 
 
 extern const char *opt_complementary;