remote-fd/ext: finishing touches after code review

remote-fd/ext: finishing touches after code review

When compiling with pthread support, transport-helper.c needs to include
necessary header files.  Also fix a few error messages in remote-ext and
remote-fd programs, and a potential buffer underrun in remote-fd.

In the documentation, clarify how %G and %V are used; the old description
looked as if they take repository/vhost parameters, which was wrong.

Also fix AsciiDoc markup for the page title of remote-fd/remote-ext manpages,
and tweak the way how section headers are shown.

Signed-off-by: Ilari Liusvaara <ilari.liusvaara@elisanet.fi>
Signed-off-by: Junio C Hamano <gitster@pobox.com>