nat: Prepare to rewrite the TP-DA number of a SMS submit.

Introduce number rewriting of SMS-SUBMIT. Introduce a new list,
move code around to help with finding a new number, somehow the
number encoding for TP-DA is borked, 03.40 references 04.11 but
the length appears to be strlen(number) without taken the type
field into account.
diff --git a/openbsc/tests/bsc-nat/bsc_nat_test.ok b/openbsc/tests/bsc-nat/bsc_nat_test.ok
index f8eee23..db37ffa 100644
--- a/openbsc/tests/bsc-nat/bsc_nat_test.ok
+++ b/openbsc/tests/bsc-nat/bsc_nat_test.ok
@@ -21,4 +21,5 @@
 Testing SMSC rewriting.
 Attempting to only rewrite the HDR
 Attempting to change nothing.
+Testing SMS TP-DA rewriting.
 Testing execution completed.