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)
test/regress: add simplestsignal: to track reorder bugs separately
Fix issue #127, double free for filterevents that use BEV_OPT_CLOSE_ON_FREE
Change return type of evutil_load_windows_system_library_ to HMODULE

Noted by Miles Chan as issue #146
Use FindClose for handle from FindFirstFile in http-server.c

Noted by Miles Chan as issue #145
Merge remote-tracking branch 'origin/patches-2.0'

Conflicts:
	sample/http-server.c
Merge pull request #153 from azat/tests-simplestsignal

test/regress: add simplestsignal: to track reorder bugs separately
Consistently check for failure from evbuffer_pullup()

Closes issue #148.
Merge remote-tracking branch 'origin/patches-2.0'
Correctly handle allocation failures in evdns_getaddrinfo

Kuldeep Gupta appears to have used some kind of automated tool to
find this and generate a patch for half the issue as #149.