milter core: Add mechanism to initialize milter-agent module
Use GMutex instead of GStaticMutex

Because GStaticMutex is deprecated since GLib 2.32.0.

This change suppresses warnings as followings:

    milter-agent.c: In function 'constructor':
    milter-agent.c:205:9: warning: 'g_static_mutex_get_mutex_impl' is deprecated (declared at /usr/include/glib-2.0/glib/deprecated/gthread.h:149): Use 'GMutex' instead [-Wdeprecated-declarations]
             g_static_mutex_lock(&auto_tag_mutex);
             ^
    milter-agent.c:209:9: warning: 'g_static_mutex_get_mutex_impl' is deprecated (declared at /usr/include/glib-2.0/glib/deprecated/gthread.h:149): Use 'GMutex' instead [-Wdeprecated-declarations]
             g_static_mutex_unlock(&auto_tag_mutex);
             ^

GitHub: see #32