#3 "cannot open server socket" should report reason

[Moritz Barsnick]

When the listening port is already bound to (in my case: an accidentally running background copy of srelay), srelay reports "cannot open server socket". I find it more enlightening if the OS reason is reported.

This can be achieved with strerror, or "%m", as used in other parts of the code.

Simple patch (from git-format-patch) attached.

[Tomo M]

serv_init() is external and %m may not be accurate error cause, but better than nothing.
thanks.