Move zonemgr to own source file

Move zonemgr to own source file

In order to make zone.c more readable, we are splitting it up in
separate source files. This moves the zonemgr to its own file
("zonemgr.c").

Since this code accesses the zone structure directly, move the
'struct dns_zonemgr' and its prerequisites to "zone_p.h".

The helper functions 'forward_cancel()', 'zone_xfrdone()',
'zmgr_start_xfrin_ifquota()', and 'zmgr_resume_xfrs() need to be
internally accessible to both source files.

Note: This commit does not compile.