minor, properly cast const char* to const void *const*

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
minor, define error() (and serror()) in dt-io.c, backlink to caller program via PROG symbol

error() will never evaluate errno.  serror() will take errno into account if != 0.

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
Merge branch 'code/hygiene'

* code/hygiene:
  minor, define error() (and serror()) in dt-io.c, backlink to caller program via PROG symbol
  minor, properly cast const char* to const void *const*