lib-sieve: include: ext-include-common - Change ext_include_get_script() into ext_include_open_script()

Implicitly open the script as well as retrieving it. This fully fixes :optional
sematics. Before, if script object creation succeeded while opening it failed
after all, :optional semantics would still cause an error.

lib-sieve: include: ext-include-common - Split off personal/global functions from ext_include_get_script()

lib-sieve: include: ext-include-common - Change ext_include_get_script_storage() into ext_include_get_script()

Later, this is needed to allow retrieving a global script from one of many
storages, rather than just one.

This makes the extension of the registered item available at evaluation time,
which is e.g. needed to access the extension context.

lib-sieve: plugins: enotify: ext-enotify-common - Rework error handling to match extension API changes

Was handled in each extension before, which is useless.

lib-sieve: plugins: environment: Use int for sieve_ext_environment_require_extension() status result

It doesn't match Dovecot convention.