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/amps/main.c

@@ -1,3 +1,4 @@
+#include <stdio.h>
 #include "main.h"
 #include "tones.h"
 #include "noanswer.h"
@@ -8,6 +9,12 @@
 const int tacs = 0;
 const int jtacs = 0;
 
+const char *number_prefixes[] = {
+	"1xxxxxxxxxx",
+	"+1xxxxxxxxxx",
+	NULL
+};
+
 int main(int argc, char *argv[])
 {
 	/* init common tones */