Clarify "Automated bounced email handler" encryption options

Affected version
v2.2.10 Patch 1
Please rename the options in the email server/credential setup for automatic bounce handling to less confusing terms, namely:

Current TLS should get renamed to STARTTLS, as STARTTLS is a) opportunistic encryption and b) not necessarily using TLS, if encrypted.

Current SSL should get renamed to SSL/TLS, as this is a non-opportunistic (required) encryption using either protocol (versions).

Small UX nit, but as one of our admins fall for that, and choose TLS for a server:port with, well actual TLS (and not STARTTLS), which resulted in a significant amount of server errors (something like "error on read due to connection close"). When reading the respective source code, it became pretty clear that this was only due to a misleading option name.

Additionally, XF could try to actually connect on saving the configuration, avoiding the case where the setup was seemingly successful only to get many errors async later on.