Added registry for extension capabilities like the available notify methods and adjusted the enotify extension accordingly.

Enabled (optional) support for Valgrind in the testsuite and fixed a few intricate bugs in the process.

Fixed bug in mail_raw implementation: mail_namespaces_deinit() must not be called explicitly for v1.2.

Previously the final value of internal variable was for every store action that didn't
specify a :flags argument explicitly. This results in out-of order assignment/removal
of flags, e.g. also the flags assigned keep actions that were executed before the
addflag/setflag command were modified.