Fixed bug in the handling of context during result execution, which resulted in broken redirect action.

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.