If stdout is a tty, use vis(3) to print any filenames to prevent garbage
from being printed if the filename contains control- or other non-printable
characters.
While here, sprinkle some EXIT_FAILURE and NOTREACHED where appropriate.
- Add strsuftoull(), which parses a number into a u_longlong_t, with
multiplication support, and support for 'g' (GB) and 't' (TB) suffices.
If an error occurs, print to stderr and exit.
Based on get_blk() from args.c and strsufto*() (in other programs)
- Add strsuftoullx(), which acts as per strsuftoull() but returns the
error in the supplied buffer instead (if the returned buffer != "", an
error occurred)
- Replace get_bsz() use with strsuftoull()
- Remove (now) unnecessary argument validation
- Remove unused {f,p,s,t}_stats fields in struct IO
(1) It won't compile, and (2) there's code in dd.c which actually
does modify (some, all, used?) conversion tables, so they can't _all_ be
const (if any can).
