Improve log message: "Security layer: requested 3, selected 1"

[buhtz]

Xrdp's log (0.9.12-1 from Debian 11 stable) often has entries like this

Security layer: requested 0, selected 1

Security layer: requested 3, selected 1

It is unclear what it means. We do not need the answer here in this Issue but in the log itself.

Please clarify the log message that it can be understood by the user/admin who reads the log messages.

[metalefty]

Indeed, that should be improved.

[matt335672]

Log messages can only be read by admins.

I've written a quick PR to log the negotiated security level as text strings as-per [MS-RDPBCGR] 2.2.1.1.1