modules: use override keyword

Resolves #736
modules: cleanup superfluous virtual keywords

These methods are not overriding anything or being overridden anywhere.
savebuff: Handle empty passwords identical

setpass <password> did not check for an empty argument.
Use the same fallback password as OnLoad()
Update Csocket to 1d7e685

> Extend EDisableProtocol
> missing ifdef for non-ssl
> added ability to disable compression
> added new callbacks for Certificate Verification and Handshake completion
> added SNI support for client and server, cleanup some of the certificate verification callback code
Fix non-SSL build
Fix #719: Disable SSL compression
Merge pull request #739 from markusj/savebuff-handle-empty-password

savebuff: Handle empty passwords identical in SetPass, and in OnLoad
configure: pass ICU build flags to modules
Merge pull request #738 from jpnurmi/override

modules: use override keyword
Merge pull request #741 from jpnurmi/charset

configure: pass ICU build flags to modules
Config option for SSL protocols (resolves #720)

ZNC currently disables SSLv2 and SSLv3 by default. To keep the ZNC
defaults (recommended, may change in the future versions) and for
example disable TLSv1 in addition, specify in the global config
section:

    SSLProtocols = -TLSv1

Available (case-insentive) values are:

    All, SSLv2, SSLv3, TLSv1, TLSv1.1, TLSv1.2

A non-prefixed "absolute" value overrides the ZNC defaults:

    SSLProtocols = TLSV1 +TLSv1.1 +TLSv1.2
Merge pull request #728 from jpnurmi/ssl-protocols

Config option for SSL protocols (#720)