For half and reverse line feeds, recognize SUSv2-style escape-digit

For half and reverse line feeds, recognize SUSv2-style escape-digit
sequences in the input stream in addition to traditional BSD-style
escape-control-char sequences because traditional tools, for example
Heirloom roff, produce SUSv2-style sequences.  Switch the encoding
of forward half line feeds in the output of -f back to SUSv2 style
because that's likely to work with more tools than the non-standard
traditional BSD-style sequence.  Fully document these choices.

Issues originally reported by bapt at FreeBSD.
OK millert@ jmc@, and bapt@ also more or less agrees with the direction.