MFC r337505, r337865, r337869: dd status=progress

r337505:
dd: add status=progress support

This reports the current status on a single line every second, mirroring
similar functionality in GNU dd, and carefully interacts with SIGINFO.

PR: 229615

r337865:
dd: Incorporate some changes from imp for status=progress

Notable changes from what landed in r337505:

• sigalarm handler isn't setup unless we're actually using it
• Humanized versions of the amount of data transferred in the progress update

r337869:
dd(1): Kill off duplicate progress definition following r337865