Remove declartion of nonexistent myerror.
Add some missing noreturn and format attributes.
Fix some format string mismatches (int versus long int).
Use an fprintf and abort instead of Nsock's fatal.

In preparation for move to Nbase.
Move CHECKED_FD_SET and CHECKED_FD_CLR to Nbase.
Use CHECKED_FD_SET and CHECKED_FD_CLR in ultra_scan.