Merging tatsuhiro-t:master -> nshoemaker:master

In reverse proxy usage, backend server most likely wants to see the
original header field.  So this commit turns off host header rewrite
by default.  --no-host-rewrite option is deprecated, and if it is
used, warning message is displayed.  --host-rewrite option is added to
enable host rewrite.

RFC 7541 requires that dynamic table size update must occur at the
beginning of the first header block, and is signaled as SETTINGS
acknowledgement.  This commit checks these conditions.  If dynamic
table size update appears other than the beginning of the first header
block, it is treated as error.  If SETTINGS ACK is received, and next
HEADERS header block does not have dynamic table size update, it is
treated as error.

When you don't generated documentation via make html