fix genkey key listing in `fdroid init`

fix genkey key listing in `fdroid init`

FDroidPopen() does not have a way to send to stdin, so we will use the
password file for now.  In the long run, at least the keypass should always
be sent via stdin rather than via a file.  Ideally, storepass would be too,
but if they are different, then storepass is less important.