Add exec: flag, 'P' (for "Priority") to bind_digit_action so that a
match is returned as soon as it is found, without waiting for the
inter-digit timeout to expire.

This can be very useful where the system needs to be more responsive
to the user.

By default, if multiple bindings are enabled and one or more use a
regex, switch_ivr_dmachine_check_match waits for the inter-digit
timeout to expire before returning a match.  This ensures overlapping
patterns, such as "^\d{4}$" and "^\12{3}$" can both be reliably matched

When the 'P' flag is specified with bind_digit_action, whose action is
exec, a match is returned as soon as the condition is satisfied,
regardless of whether or not a longer match may be possible if further
digits were entered.

For example:

	<action application="bind_digit_action" data="test,~^*\d{2}$,
		exec[P]:execute_extension,myextn,self,self"/>

	<action application="bind_digit_action" data="test,~^#\d{2}$",
		exec:execute_extension,myotherextn,self,peer"/>

	The first example causes a match to be returned immediately
	after the 2nd digit is received, whereas the second example
	defaults to waiting for the inter-digit timeout to expire
	before returning.

In cases where the 'P' flag is used with a regex and string, and both
are matched, the more explicit, string match will be returned.

For example:

	<action application="bind_digit_action" data="test,~^*\d{2}$,
		exec[P]:execute_extension,myextn,self,self"/>

	<action application="bind_digit_action" data="test,*12,
		exec[P]:execute_extension,myotherextn,self,self"/>

	If "*12" is matched, myotherextn is executed, because "*12" is
	more explicit/specific than "^*\d{2}$"

If the 'P'(riority) flag is not used, behaviour is unchanged from
previous versions.  This ensures backward compatibility.

FS-9325 #resolve