Fix a segfault in lash, hush, and cmdedit. Each of these used

xgetcwd, but did not check the return for a NULL, and then continued
to call strlen on the NULL when the cwd had been removed from under it.
 -Erik

include/libbb.h

@@ -282,5 +282,6 @@ extern const char * const dash_dash_help;
 extern const char * const write_error;
 extern const char * const too_few_args;
 extern const char * const name_longer_than_foo;
+extern const char * const unknown;
 
 #endif /* __LIBBB_H__ */