copy first result to preallocated buffer

Fixes a segfault in nscd when using nss-myhostname.

Nscd expects that an NSS module's gethostbyname4_r function returns
its first result in the pre-allocated gaih_addrtuple denoted by **pat.
(See nscd/aicache.c in the Glibc sources.)  However, nss-myhostname
doesn't fill in **pat but allocates the first result in ‘buffer’, then
sets *pat.  So nscd crashes (e.g. when running ‘getent ahosts
my-machine’).

Hard to tell if this is a bug in nscd, since there doesn't seem to be
a proper API spec for gethostbyname4_r.  But in any case, this patch
fixes the crash by copying the first result to **pat.
Add autopkgtest to test hostname resolution without /etc/hosts, and also test with nscd installed.
merge patched into master
releasing package libnss-myhostname version 0.3-7