ssl: use secure socket
ssl: use client key/certificate, fallback to plaintext

To connect with TLS/SSL, APRSdroid is looking for a file called
app_keystore/CALLSIGN.bks in its private directory (file deployment is
still TBD).

The key store needs to be secured using the password "APRS-IS" and
contain the following:

 * a private key for this client (key password: "APRS-IS")
 * a certificate for that key, with OID 1.3.6.1.4.1.12348.1.1=CALLSIGN)
 * a chain of trust for this certificate, leading to a trusted root
ssl: display name and callsign
ssl: improve fallback to plaintext (untranslated txt)
ssl: implement import of .bks files
Switch from BKS to PKCS12 (like LotW export)