stdvgaio: Only read/write one color palette entry at a time

stdvgaio: Only read/write one color palette entry at a time

Introduce stdvga_dac_read_many() and stdvga_dac_write_many() for
writing multiple dac palette entries.  Convert the stdvga_dac_read()
and stdvga_dac_write() low-level IO access functions in stdvgaio.c to
access just one color palette entry.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>