avutil/opt: Add AV_OPT_TYPE_UINT64

Requested-by: wm4 ([FFmpeg-devel] [PATCH] avutil/opt: Support max > INT64_MAX in write_number() with AV_OPT_TYPE_INT64)
Requested-by: ronald ([FFmpeg-devel] [PATCH] avutil/opt: Support max > INT64_MAX in write_number() with AV_OPT_TYPE_INT64)
Reviewed-by: Andreas Cadhalpun <andreas.cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>