Improved various aspects of the plugin-related extension API.

src/lib-sieve/plugins/enotify/ext-enotify-common.c

src/lib-sieve/plugins/enotify/ext-enotify.c

src/lib-sieve/plugins/enotify/sieve-ext-enotify.h

src/lib-sieve/sieve-extensions.c

src/lib-sieve/sieve-extensions.h

src/lib-sieve/sieve-plugins.c

src/lib-sieve/sieve.c