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 ch...