support Apache’s REDIRECT_REMOTE_USER CGI env var

* In places where the CGI variable REMOTE_USER is read, support alternatively
  REDIRECT_REMOTE_USER, which is used by the Apache HTTPD Server instead, when a
  redirect was used.

Note: This alone is not enough yet, to fully support it in DAViCal. An analogous
      change (commit 29ddd89baaf65bda2560e51665a2e761abef4147) is necessary in
      AWL.