app_mf: Add max digits option to ReceiveMF.

Adds an option to the ReceiveMF application to allow specifying a
maximum number of digits.

Originally, this capability was not added to ReceiveMF as it was
with ReceiveSF because typically a ST digit is used to denote that
sending of digits is complete. However, there are certain signaling
protocols which simply transmit a digit (such as Expanded In-Band
Signaling) and for these, it's necessary to be able to read a
certain number of digits, as opposed to until receiving a ST digit.

This capability is added as an option, as opposed to as a parameter,
to remain compatible with existing usage (and not shift the
parameters).

ASTERISK-29877 #close

Change-Id: I4229167c9aa69b87402c3c2a9065bd8dfa973a0b