Document compiling apps and include h2load in configure

This makes linking fail on -ldl as there is no libdl on in /usr/lib or /usr/local/lib on *BSD
Tested (and part of the proposed nghttp2 1.7.0) on [https://reviews.freebsd.org/D5218|FreeBSD]

This commit adds request-header-field-buffer and
max-request-header-fields, and deprecates header-field-buffer and
max-header-fields options.

This change removed client::session::connect_timeout() functon, and
connect timeout should be given to client::session constructor.  This
is required since async operation starts in the constructor.

Evaluate nghttp2_session_want_read and nghttp2_session_want_write when determining session is about to close