Force Mac OS shared library extension to .so

Darwin and libtool seem confused about what shared library
extension they actually use.

https://bugs.freedesktop.org/show_bug.cgi?id=57714
Fix uninitialized p11_library_once

https://bugs.freedesktop.org/show_bug.cgi?id=57714
trust: Fix reinitialization of trust module

Track number of C_Initialize calls, and require similar number
of C_Finalize calls to finalize.

This fixes leaks/disappearing sessions in the trust module.

https://bugs.freedesktop.org/show_bug.cgi?id=65401
trust: Fix crash when C_Initialize args are NULL

https://bugs.freedesktop.org/show_bug.cgi?id=65401
Release version 0.18.3
Merge branch 'stable'