Fix some problems on systems where EAGAIN != EWOULDBLOCK (e.g., OS/390).

ap_canonical_errror() for Unix now maps EWOULDBLOCK to EAGAIN/APR_EAGAIN
when appropriate so that the changes here (as well as bjh's changes from a
few days ago) to call ap_canonical_error() before comparing with EWOULDBLOCK
will work on Unix as well.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@85019 13f79535-47bb-0310-9956-ffa450edef68