Refactoring validity check and prefix processing of dialed number

Command line help shows how many digits and what prefixes can be dialed.

Giving a station ID via command line will be checked for being valid.

The number to call the mobile statione will be checked for being valid.

Prefixes that are defined for a nework will be removed from station ID
automatically.

Multiple station ID lengths are supported:
 * C-Netz: 7 or 8 digits, depending on area code length
 * A-Netz: 5 or 7 digits; number is truncated to last 5 digits.
 * IMTS/MTS: 5 or 7 digits, depending on phone's selector switch.

src/cnetz/cnetz.h

@@ -133,6 +133,7 @@ struct cnetz {
 	transaction_t		*trans_list;		/* list of transactions */
 };
 
+const char *cnetz_number_valid(const char *number);
 double cnetz_kanal2freq(int kanal, int unterband);
 void cnetz_channel_list(void);
 int cnetz_channel_by_short_name(const char *short_name);