Integrate Debian packaging changes

debian/control:
* restructure to make it easier to incorporate further changes
* update package descriptions
* move build-depends to a proper place
* update project URL

debian/rules:
* use proper hardening syntax
* strip linker option without explicit shell invocation
* remove useless comment
* add extra cleanup

debian/: package documentation separately

debian/docs: remove empty file

debian/coryright: update to match Debian format

Change-Id: Ia7654d34730e9f269831612bfba70a1338ce29d3
Related: OS#1694
diff --git a/debian/libosmogsm-doc.install b/debian/libosmogsm-doc.install
new file mode 100644
index 0000000..a4300bc
--- /dev/null
+++ b/debian/libosmogsm-doc.install
@@ -0,0 +1 @@
+usr/share/doc/libosmocore/gsm/