Drop the ErrorHeader directive which turned out to be a misnomer.

Drop the ErrorHeader directive which turned out to be a misnomer.
Instead there's a new optional flag for the Header directive
('always'), which keeps the former ErrorHeader functionality.

The opposite flag to 'always' is 'onsuccess'. So the new directives are written
as:

Header set foo bar
# which is the same as
Header onsuccess set foo bar
# and a subset of
Header always set foo bar
# which is what the ErrorHeader directive did.

Reviewed by: Brad Nicholes

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@103920 13f79535-47bb-0310-9956-ffa450edef68