component/userhosts: use github.com/figiel/hosts for compatibility with recent libc

In ubuntu 21.04 (libc6 2.33-0ubuntu5) or current debian testing
(libc6 2.32-5) no longer use open to open /etc/hosts, but what appears
as "openat" in strace output - but can not be replaced by defining an
openat function.

This uses https://github.com/figiel/hosts which uses another approach
of replacing getaddrinfo, gethostbyname* and inet_aton.

Users have been updated a bit, because there are some small differences:
 - the /etc/hosts replacement file is defined by HOSTS_FILE environment
   variable, not HOSTS
 - the library name is libuserhosts.so, not userhosts.so

Other notable differences, for which we did not need code change are:
 - the new library also try to load a file when HOSTS_FILE is not set
 - the new library still use original /etc/hosts file
 - the new library supports aliases to hostnames, not only ip addresses