Merge branch 'http-connect'

After this patchset http-connect works with pproxy [1]:

$ pproxy -l http://:8000/ -vvv &
$ http-connect //127.1:8000 http://kernel.org:80/

[1]: https://pypi.o

Merge branch 'http-connect'

After this patchset http-connect works with pproxy [1]:

$ pproxy -l http://:8000/ -vvv &
$ http-connect //127.1:8000 http://kernel.org:80/

[1]: https://pypi.org/project/pproxy/

* http-connect:
http-connect: do not check connection on GET cb
http-connect: set Host header (for CONNECT and GET) (like curl)
http-connect: cleanup and helpers
http: do not close connection for CONNECT
http: do not assume body for CONNECT

Fixes: #946
(cherry picked from commit 462f2e97660cc2b6af563ab631050a83c3e0935b)

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http-connect: exit on error

(cherry picked from commit d22e12eefa11aab34b95e8b07b823f3ad0597239)

sample/http-connect: fix compilation with VS2010

Fixes: nmathewson/Libevent#137

http-connect: make it win32 compilable

Fixes: commit 1d34498ea31134bd7a8fcadcb95910b30a479e19 ("sample: add HTTP
CONNECT tunnelling example using libevent http layer")

sample/http-connect: don't use assert() to make it work with NDEBUG

Otherwise it will not work for cmake release target type.

sample/http-connect: made it compatible with C90

sample: add HTTP CONNECT tunnelling example using libevent http layer