Introduce libsmpputil

As part of preparation for libosmo-netif migration let's move common SMPP code
into separate build-time library and use it for both smpp_mirror and OsmoMSC
renaming the files if necessary.

While at it we also fix id/password legth limits in smpp_mirror and drop unused
fields from ESME struct.

Related: OS#5568
Change-Id: I61910651bc7c188dc2fb67d96189a66a47e7e8fb
diff --git a/include/osmocom/Makefile.am b/include/osmocom/Makefile.am
index 4d80637..b07a004 100644
--- a/include/osmocom/Makefile.am
+++ b/include/osmocom/Makefile.am
@@ -1,3 +1,4 @@
 SUBDIRS = \
 	msc \
+	smpp \
 	$(NULL)