Add shared subtree support, suggested by Vladimir Dronnikov. Also break out a

few new (unfinished) config options, which I intend to make hidden (but
enabled) when CONFIG_NITPICK is disabled.  Getting the .config infrastructure
to do that is non-obvious, it seems...

include/libbb.h

@@ -407,7 +407,7 @@ extern const char * const bb_default_login_shell;
 #define DEFAULT_SHELL_SHORT_NAME     (bb_default_login_shell+6)
 
 
-extern const char bb_path_mtab_file[];
+extern char bb_path_mtab_file[];
 
 extern int bb_default_error_retval;