Add die(fmt, ...). Equivalent to adios(NULL, fmt, ...).

Add die(fmt, ...).  Equivalent to adios(NULL, fmt, ...).

Avoids the noise of the `NULL' first parameter that's used in over 70%
of adios() calls.  Removes the possibility of it being omitted and `fmt'
being used instead.  `die' is already in use in nmh's shell scripts.
Had to rename post.c's existing die() to avoid it clashing.