Implement account login support

This is done via the `accountname' METADATA command and used to
automatically identify users after netsplits or across service
restarts.
Plug memory leak
Silence warning

Cast the result of the operation to long, not the time(NULL) call.
On systems where sizeof(time_t) is other than long this will produce
a warning.
Update METADATA commands

Document accountname, update certfp details and sort entries.