xbps-rindex(8): added support for XBPS_PASSPHRASE env var.

- XBPS_PASSPHRASE (if set) will be supplied as passphrase when signing repos.
- Use the default OpenSSL passphrase cb when loading the private key.

bin/xbps-rindex/defs.h

@@ -74,9 +74,6 @@ int	remove_obsoletes(struct xbps_handle *, const char *);
 int	sign_repo(struct xbps_handle *, const char *, const char *,
 		const char *);
 
-/* From readpass.c */
-char	*readpassphrase(const char *, char *, size_t, int);
-
 /* From repoflush.c */
 bool	repodata_flush(struct xbps_handle *, const char *,
 		xbps_dictionary_t, xbps_dictionary_t, xbps_dictionary_t);