lib/ucs2_string: Add UCS-2 strscpy function

Add a ucs2_strscpy() function for UCS-2 strings. The behavior is
equivalent to the standard strscpy() function, just for 16-bit character
UCS-2 strings.

Signed-off-by: Maximilian Luz <luzmaximilian@gmail.com>
Reviewed-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230827211408.689076-2-luzmaximilian@gmail.com
Signed-off-by: Bjorn Andersson <andersson@kernel.org>