Fix the use of the default 'flush' provider.

Improve documentation for the "flusher" parameter.
Remove useless empty lines.

See http://mail-archives.apache.org/mod_mbox/httpd-dev/200812.mbox/%3C494226C0.4050407@force-elite.com%3E for some more explanation.
A python script is given there to test.
I had to tweak it to have it work
(use:
        fd, payload = passfd.recvfd(conn.fileno())
instead of:
        fd = passfd.recvfd(conn.fileno())
)


This is a r1058621 regression, where somehow "char *flusher" has been turned into a "char flusher[]". So it is been broken since the beginning of 2.4.x

After this change (i.e. r1058621), 'flusher' is no more a pointer (NULL'ed when the structure it belongs to is created) but the address of an array within a structure. It can not be NULL anymore.
So, we now have to look at the content of the array itself to see if it has been initialized or if we have to use the default value instead.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1717063 13f79535-47bb-0310-9956-ffa450edef68