bugfix: fwmark field was formatted as signed int
dump_conn_opts: fwmark was not displayed
log_message: emit -Wformat= compiler warnings

There could be (and actually are) situations when the format string and
the arguments list passed to the log_message() are inconsistent or mistyped.
The compiler did not show any warnings because the vsnprintf was called
indirectly.
fix -Wformat= warnings in log_message() calls
Further unification of IP endpoints logging

This change tries to keep usage of the standard "[%s]:%d" format string to a minimum.
Instead, use inet_sockaddrtopair wherever possible.