librpc: Change type of ‘u16string’ from ‘const uint16_t *’ to ‘const unsigned char *’

librpc: Change type of ‘u16string’ from ‘const uint16_t *’ to ‘const unsigned char *’

A u16string is supposed to contain UTF‐16 code units, but
ndr_pull_u16string() and ndr_push_u16string() fail to correctly ensure
this on big‐endian systems. Code that relies on the u16string array
containing correct values will then fail.

Fix ndr_pull_u16string() and ndr_push_u16string() to work on big‐endian
systems, ensuring that other code can use these strings without having
to worry about first encoding them to little‐endian.

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