Add evhttp_connection_set_family() to set addrinfo->family for DNS requests

This is useful if you want to avoid extra dns requests.
Load hosts file on Windows.

Hosts files are not loaded in evdns_base_config_windows_nameservers() if
load_nameservers_with_getnetworkparams() succeeds on Windows. Parse and
load it first before setting up nameservers.
Don't truncate hosts file path on Windows.

Since evutil_snprintf() (actually evutil_vsnprintf() called by it) will
make sure the buffer is null-terminated by placing a null byte at
len_out - 1, we need to pass the full length of the buffer; otherwise
the path will end in "\\host" instead of "\\hosts".
test: add family argument for http_connection_test_()
test: add regress for evhttp_connection_set_family() with AF_INET and AF_UNSPEC
test/http: add regress test for set family to AF_INET6
evdns: disable probing with EVDNS_BASE_DISABLE_WHEN_INACTIVE

When user install EVDNS_BASE_DISABLE_WHEN_INACTIVE flag for evdns base,
we must remove the timer that is used for probing, if current dns server
failed, otherwise it won't break the loop.
evdns: add regress for EVDNS_BASE_DISABLE_WHEN_INACTIVE base flag
evdns: add regress test for flag EVDNS_BASE_DISABLE_WHEN_INACTIVE without ns
evdns: we don't need to add retry timer in transmit_to it will be done on fail
regress dns: drop alarm() (since windows don't have it)
make bufferevent_getwatermark api more robust
Merge pull request #129 from azat/disable-probing-with-disable-when-inactive-v2

evdns: disable probing with EVDNS_BASE_DISABLE_WHEN_INACTIVE
Merge pull request #128 from azat/tests-disable-when-inactive-v5

Some tests for EVDNS_BASE_DISABLE_WHEN_INACTIVE flag
use correct tt macro for pointer compare
Merge pull request #118 from azat/http-forward-family-to-bufferevent

Add evhttp_connection_set_family() to set addrinfo->family for DNS requests
regress_buffer: fix 'memcmp' compare size
Merge remote-tracking branch 'origin/patches-2.0'
Fix use-after-free error in EV_CLOSURE_EVENT callback
Merge pull request #119 from ldx/winhosts

Fix hosts file use on Windows.