Change the ap_cfg_getline() and ap_cfg_getc() to return an error code.

Change the ap_cfg_getline() and ap_cfg_getc() to return an error code.

Also:
- Make ap_cfg_getline() return APR_ENOSPC if a config line is too long.
- Add ap_pcfg_strerror() function to convert ap_cfg_getline's return value
  into a nice message.
- Adjust definition of ap_configfile_t accordingly.

Not bumping MMN because it has already been bumped today.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1086756 13f79535-47bb-0310-9956-ffa450edef68