digsig: make debug messages more elegant

Remove ugly all-caps prefix tag from debug messages, and reorganize
dsi.h to logically group the debug print macros together.

Signed-off-by: Rupert Muchembled <rupert@rmuch.net>
digsig: rename dsi.h to digsig_common.h

Rename dsi.h to digsig_common.h, to better signify its purpose as a
header for common inclusions and debug macros.

Signed-off-by: Rupert Muchembled <rupert@rmuch.net>