Rework cloaked hostname handling, implement "METADATA cloakhost"

Rework cloaked hostname handling, implement "METADATA cloakhost"

Now ngIRCd uses two fields internally, one to store the "real" hostname
and one to save the "cloaked" hostname. And both fields can be set
independently using the "METADATA host" and "METADATA cloakhost" commands.

This allows "foreign servers" (aka "IRC services") to alter the real and
cloaked hostnames of clients without problems, even when the user itself
issues additional "MODE +x" and "MODE -x" commands.