Merge r1802040, r1807876, r1808014, r1805490, r1823886 from trunk:

Merge r1802040, r1807876, r1808014, r1805490, r1823886 from trunk:

mod_proxy_fcgi: Add the support for mod_proxy's
                flushpackets and flushwait params

This change was requested on the development mailing
list in order to fill another gap between mod_fcgi
and mod_proxy_fcgi, namely the -flush funtionality.

The more evolved core trunk code would not need this
feature becuse of the non-blocking writes, but it
is be needed in 2.4.x.

mod_proxy_fcgi: limit the flush buckets inserted when flushpackets=on|auto

This commit is a follow up of r1802040 based on Jacob's
feedback, namely inserting the FLUSH buckets only when
really needed and useful, not always.

mod_proxy_fcgi: follow up to r1807876.
Fix mixed declarations and code [-Wdeclaration-after-statement].

Fix a compilation warning introduced by r1802040.
mod_proxy_fcgi.c:893:19: warning: ‘flushpoll’ may be used uninitialized in this function [-Wmaybe-uninitialized]

This warning is a false positive.

mod_proxy_fcgi: prioritize the check for mayflush when using flushpackets

The mayflush variable should be checked before the rest
to avoid polling when not needed.

Suggested by Yann Ylavic on the dev@ mailing list.

Submitted by: elukey, ylavic, jailletc36, elukey
Reviewed by: elukey, jim, ylavic

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1825765 13f79535-47bb-0310-9956-ffa450edef68