Add m_dccblock module to completely block DCC xfers and DCCALLOW exceptions

I wanted to share a module I made for inspircd 2.0.x in case you are interested in taking it upstream for wider availability.  It is called m_dccblock and is adapted from m_dccallow but applies a unilateral block to all DCC/DCCALLOW requests.  The original m_dccallow module could be configured to disable allowing transfers by default, but had no way to turn off the DCCALLOW exception list so users could potentially be socially engineered into enabling DCC.  For people running IRC servers in communities where the users highly value privacy and security, sometimes a complete disabling of DCC is preferred, and it's safer to do this in a new module with most of the functionality stripped out (in my opinion) than adding more functionality to m_dccallow to try to disable it further conditionally.  The module has been simplified as much as possible, so, for instance, it just gives an error message indicating DCC is not allowed to the user if they try to send a file, instead of just giving a message about that particular user not accepting files.  DCCALLOW gives the same error.