Changes

libgtp: move includes to osmocom/include/gtp

Move all includes from /usr/include/….h to
/usr/include/osmocom/gtp/….h to be more consistent with other Osmocom
projects, and to not "pollute" the top include directory if we add more
header files.

Also the new directory structure makes more obvious, which headers are
public and which ones aren't.

Adjust libgtp.pc.in so both #include <gtp.h> (legacy)
and #include <osmocom/gtp/gtp.h> can be used.

Related: OS#6373
Change-Id: If7e01c61168819bf7120667344e40c857da5490b

gtp/gsn_internal.h: new file

Change-Id: I999462e39411fc4ec7e50bd0212e870006fbc4f1

gtp/gtp.c: move gtp_create_context_resp down

Move gtp_create_context_resp below gtp_create_pdp_resp, which it calls.
In a follow-up commit, we can make gtp_create_pdp_resp static and
remove the additional declaration.

Change-Id: I34efe7592013a8423f4f280758272d81f24b65fa

gtp: remove unused conversion functions

Remove ipv42eua and eua2ipv4, which are in the "internal functions"
section of gtp.h, but are not used anywhere in the code anymore. This is
in preparation of moving the internal functions that are used in
multiple .c files into a separate header file, and to make the other
internal functions static. (Compiler complains about unused static
functions.)

Change-Id: I90e2750f6a6e3e6122e9c562103fda77d7326932

gtp: move conversion functions up

Move the conversion functions above the first user, so we can make
in_addr2gsna static in a follow-up commit and remove the extra
declaration.

Change-Id: I51e6a7c1161320fc54b0e8197ae57e4327976eb1

gtp/gtp_internal.h: new file

While at it, move internal functions of gtp.h to a separate file too.
Make all functions that are only used inside gtp.c static.

The following APIs are unexpectedly public:
* imsi_gtp2str gets used by pdp.h
* gsna2in_addr, gtp_echo_req gets used by osmo-sgsn

Change-Id: I72c40cbdec33449ca8104fb3cad8df1a9e07dfd7