Enhance SSL/TLS configuration options

New settings:

    tls_compression: 0

        Enable TLS compression. Disabled by default.

    tls_eccurve: prime256v1

        Select the elliptic curve used for ECDHE.

    tls_prefer_server_ciphers: 0

        Prefer the cipher order configured on the server-side.

    tls_versions: ssl2 ssl3 tls1_0 tls1_1 tls1_2

        Disable SSL/TLS protocols not in this list.

Bugzilla #3822, #3830, #3843, #3861.
lmtpd.c: Fix for bug #3856: lmtpd now obeys user_deny.db