move kern_unveil.c to use DPRINTF()

move kern_unveil.c to use DPRINTF()

Changes the way printf debug is done in kern_unveil.c

Currently, each printf() is enclosed in #ifdef DEBUG_UNVEIL. It moves
to using DPRINTF(), and reduces the number of #ifdef inside the file.

Also changes some strings to use __func__ instead of using the
function name verbatim.

ok visa@