* Added valgrind memory leak tests in testmem.sh for restconf

* Added clicon_socket_set() and clicon_socket_get() functions for cleaning up backend server and restconf FCGI socket on termination.
* Removed obsolete `-x` command-line option to clixon_cli.

test/valgrind-clixon.supp

@@ -0,0 +1,44 @@
+{
+   supp1
+   Memcheck:Leak
+   match-leak-kinds: reachable
+   fun:*
+   fun:_dl_new_object
+}
+{
+   supp2
+   Memcheck:Leak
+   match-leak-kinds: reachable
+   fun:*
+   fun:*
+   fun:expand_dynamic_string_token
+}
+{
+   supp3
+   Memcheck:Leak
+   match-leak-kinds: reachable
+   fun:*
+   fun:_dlerror_run
+}
+{
+   supp4
+   Memcheck:Leak
+   match-leak-kinds: reachable
+   fun:*
+   fun:_dl_check_map_versions
+}
+{
+   supp5
+   Memcheck:Leak
+   match-leak-kinds: reachable
+   fun:*
+   fun:*
+   fun:_dl_map_object
+}
+{
+   supp6-fcgi
+   Memcheck:Leak
+   match-leak-kinds: reachable
+   fun:*
+   fun:OS_LibInit
+}