debug: use gai_strerror() on getnameinfo() failure

getnameinfo() returns an error code which can be passed to
gai_strerror() for human consumption.  So use that instead of
strerror(errno) unless the code is EAI_SYSTEM.

Signed-off-by: Julien Cristau <jcristau@debian.org>
Cc: Philipp Kern <pkern@debian.org>
debian/changelog: add entry for 28d5f4c
update netcfg