Implemented issue #1015: Set display_errors and error_reporting to immutable values.

I've added the xdebug.force_display_errors setting to force the display of
errors. I've also added the xdebug.force_error_reporting setting which acts
like a bitmask of errors that are force to be on. So that means that
xdebug.force_error_reporting=E_WARNING|E_USER_WARNING will make Xdebug *always*
display those two error types, regardless of what ini_set('error_reporting',
…); does. You can use xdebug.force_error_reporting=-1 to force the display of
all errors. xdebug.force_error_reporting can also be set through ini_set(),
although I perhaps will disable that later (to prevent "clever" scripts).
Fixed a missing function call name change.
Merged pull request #99