Use getaddrinfo and parse /etc/hosts on linux

[gittywithexcitement]

The motivation for this change is that my application needs /etc/hosts to be parsed and used.
So I changed usage of evdns_base_resolve_ipv4/ipv6 to getaddrinfo, because evdns_base_resolve_XXX doesn't use the hosts file.

You may be wondering why I then applied a commit "Change EventDriver.resolveHost parameter..." The answer is that I can't compile the current master HEAD (see #458). So I branched from the most recent commit that compiles for me, and made my changes. Then I cherry picked d49f615 and resolved conflicts.

So far I've only tested with an /etc/hosts containing localhost. Tomorrow I will test with a more challenging /etc/hosts file that maps nameFoo to 127.0.0.1 and nameBar to a remote IP. I have every reason to believe it will work.

[s-ludwig]

This is great! I didn't know that evdns_base_load_hosts exists (really should have scanned through the whole API once). So this fixes #289. your commit is still based on the old commit. I'll try to cherry pick the actual change into master and test.

[gittywithexcitement]

Note that for windows, hosts file is in \windows\system32\drivers\etc\hosts. I wasn't sure how to get that path in the canonical way, so I didn't implement it.

[gittywithexcitement]

Actually, rather than base_load_hosts, it's probably better to call
int evdns_base_resolv_conf_parse(struct evdns_base *base, int flags, const char *filename);
With flags = DNS_OPTIONS_ALL or flags=DNS_OPTION_HOSTSFILE at the very least. I've never used resolv.conf, but this path seems the most 'full featured'.
See http://www.wangafu.net/~nickm/libevent-book/Ref9_dns.html It points out that this is a linux feature, and for windows, evdns_base_config_windows_nameservers applies. As a side benefit, it looks like evdns_base_config_windows_nameservers will do the dirty work of finding the location of Windows' nameserver settings.
Anyway, base_load_hosts is good enough for my current purposes, but I thought I'd point out resolv_conf_parse

[s-ludwig]

I've hard-coded the "C:\Windows..." path as a first step and will just use the %SYSTEMROOT% env var to get it correctly.

Thanks for the additional evdns_ suggestions. I'll look into it.

[s-ludwig]

This PR was already integrated into master manually and can be closed.