Bug 10277 - Add C4::Context->IsSuperLibrarian()

Bug 10277 - Add C4::Context->IsSuperLibrarian()

The method of checking the logged in user for superlibrarian privileges
is obtuse ( $userenv && $userenv->{flags} % 2 != 1 ) to say the least.
The codebase is littered with these lines, with no explanation given. It
would be much better if we had one subroutine that returned a boolean
value to tell us if the logged in user is a superlibrarian or not.

Test Plan:
1) Apply this patch
2) Verify superlibrarian behavior remains unchanged

Signed-off-by: Joel Sasse <jsasse@plumcreeklibrary.net>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Comments on second patch.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>