Add inter-thread queue

This adds an inter-thread queue "it_q" to libosmocore. With it_q,
one can perform thread-safe enqueing of messages to another thread,
who will receive the related messages triggered via an eventfd
handled in the usual libosmocore select loop abstraction.

Change-Id: Ie7d0c5fec715a2a577fae014b0b8a0e9c38418ef
diff --git a/tests/testsuite.at b/tests/testsuite.at
index ad93e16..75ce039 100644
--- a/tests/testsuite.at
+++ b/tests/testsuite.at
@@ -427,3 +427,9 @@
 cat $abs_srcdir/bssmap_le/bssmap_le_test.ok > expout
 AT_CHECK([$abs_top_builddir/tests/bssmap_le/bssmap_le_test], [0], [expout], [ignore])
 AT_CLEANUP
+
+AT_SETUP([it_q])
+AT_KEYWORDS([it_q])
+cat $abs_srcdir/it_q/it_q_test.ok > expout
+AT_CHECK([$abs_top_builddir/tests/it_q/it_q_test], [0], [expout], [ignore])
+AT_CLEANUP