Fix epmd's dbg_perror() output

[Holger Weiß]

When epmd prints an error message via dbg_perror(), the dbg_gen_printf()
function is supposed to append the error message which corresponds to
the current errno value. The following commit fixes the problem that
errno has been reset to zero (by erts_snprintf()) before it is used:

    git fetch git://github.com/weiss/otp.git fix-epmd-perror

The problem can easily be reproduced by executing e.g.

    epmd -port 42

as an unprivileged user. Without the patch, this will spit out something
along the lines of:

    epmd: Wed Nov 17 16:44:22 2010: failed to bind socket: Success

Holger