Allow user mode +x only when "CloakHostModeX" is set

Allow users to "cloak" their hostname only when the configuration
variable "CloakHostModeX" (introduced in 19.2) is set. Otherwise, only
IRC opertators, other servers, and services are allowed to set mode +x.

This prevents regular users from changing their hostmask to the name
of the IRC server itself, which confused quite a few people ;-)

This fixes bug #133.