default to inheriting O_NONBLOCK?

At the moment APR penalizes Linux[1] compared to FreeBSD by never 
passing SOCK_NONBLOCK in apr_socket_accept()'s accept4() calls. This is 
annoying for httpd, which always sets O_NONBLOCK for the accepted socket 
anyway - at the cost of two extra system calls.

Thanks to r1551659 [2] if you build APR *pretending* that O_NONBLOCK is 
inherited, it actually *will* be inherited by apr_socket_accept(), by 
enabling use of SOCK_NONBLOCK.

  ./configure ... ac_cv_o_nonblock_inherited=yes

Does anybody see any reason why we couldn't build APR like this by 
default (...for Linux)? httpd never checks APR_O_NONBLOCK_INHERITED at 
build time. I did a test httpd CI run, changing the APR builds like this 
where possible, and it didn't make any difference. [3]

An alternative would be to add an apr_socket_accept2()/_ex() which 
allows passing flags, which httpd could use to force use of 
SOCK_NONBLOCK where available.

Regards, Joe

[1] Strictly: on Unix platforms which don't inherit O_NONBLOCK across 
accept() but do have the accept4() system call

[2] https://github.com/apache/apr/commit/100101fd3f1e105144fec2cd7e81b8793cca2693

[3] https://github.com/apache/httpd/pull/445 - note that not all the 
jobs build APR from source, rather using the Ubuntu system version