app_sf: Add post-digit timer option to ReceiveSF.

app_sf: Add post-digit timer option to ReceiveSF.

Add a sorely needed option to set a timeout between digits, rather than
for receiving the entire number. This is needed if the number of digits
being sent is unknown by the receiver in advance. Previously, we had
to wait for the entire timer to expire.

Resolves: #1493

UserNote: The 't' option for ReceiveSF now allows for a timer since
the last digit received, in addition to the number-wide timeout.