Show connection flag "s" (SSL) in RPL_TRACE{LINK|SERVER}

Now you can check if a server-to-server link is SSL-encrypted or not
using the IRC "TRACE" command.

Idea by Götz Hoffart, thanks!
ngircd: use setgid/setuid errno value in error path

Need to use saved errno value as strerror argument, else you
get bogus output ('success') in the log message.