Combine large bwrite() with partial buffer by using writev() to avoid any

mem->mem copies.  Fix a bug where if bflush/bwrite had a write failure
while chunking they won't properly start the next chunk.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@78810 13f79535-47bb-0310-9956-ffa450edef68