dirmngr: Resurrect hosts in the HKP hosttable.
* dirmngr/dirmngr.c (HOUSEKEEPING_INTERVAL): New. (housekeeping_thread): New. (handle_tick): Call new function. * dirmngr/ks-engine-hkp.c (RESURRECT_INTERVAL): New. (struct hostinfo_s): Add field died_at and set it along with the dead flag. (ks_hkp_print_hosttable): Print that info. (ks_hkp_housekeeping): New.
The resurrection gives the host a chance to get back to life the next
time a new host is selected.