Add FALSE() function

More convenient than writing ' === FALSE' everywhere.
ZarafaCardDavBackend: getCard(): add local cache

Speed up the time it takes to list a large contacts dir by caching the lookups.
SabreDAV calls this function twice for each card.
ZarafaPrincipals: simplify code

Radically simplify the code by stripping unnecessary variables, assignments,
indents, spaces, code duplication, and so on.