kern/misc, kern/efi: Extract UTF-8 to UTF-16 code

kern/misc, kern/efi: Extract UTF-8 to UTF-16 code

Create a new function for UTF-8 to UTF-16 conversion called
grub_utf8_to_utf16_alloc() in the grub-code/kern/misc.c and replace
charset conversion code used in some places in the EFI code. It is
modeled after the grub_utf8_to_ucs4_alloc() like functions in
include/grub/charset.h. It can't live in include/grub/charset.h,
because it needs to be reachable from the kern/efi code.

Add a check for integer overflow and remove redundant NUL-termination.

Signed-off-by: Oliver Steffen <osteffen@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>