replace problematic hostentdb with hostsreader

the hostentdb introduced between 4.2 and 4.3
(via af5c6f0c6a7cddd46b801b7b59af7a5e9383c645 )
had several issues:
- it caused breakage on FreeBSD and was commented out there
- prevented usage of the hostdb when proxy_dns was turned off
  (issue #42)
- required dynamic memory allocation which was accessed from several
  threads
- wouldnt reflect changes to the hosts file made during program run

the only sensible solution is to remove the hostentdb and replace it
with a home-grown hosts parser (we can't use gethostent() since
that would mess up the gethostent()-state from different threads).

the new parser used here is deliberately held simple and only meant
to provide the user with means to reference hardcoded ipv4 addresses
via his hosts file.

fixes #42