pidl: Add new ‘u16string’ type

pidl: Add new ‘u16string’ type

This type represents a UTF‐16–encoded string. These strings are kept
UTF‐16–encoded rather than converted to the Unix charset to be stored in
memory; this avoids issues regarding NULL termination and conversion
between character sets. We want to be able to handle strings that are
not valid UTF‐16.

Not bumping the NDR ABI version, because there hasn’t been an NDR
release since commit c4f281e9ae36c225b6003e0fa1cb8fb2e67bf543.

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>