Used user event before. This will be needed for retrieving settings in mailbox
context.

It just returns the one storage that the provided location points to for now.

lib-sieve: sieve-storage - Split off sieve_storage_save_is_activating_default() from sieve_storage_save_commit()

lib-sieve: sieve-storage - Change storage_class parameter of sieve_storage_create_event() into driver_name

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.

lib-sieve-tool: sieve-tool - Rename filename to location parameter for sieve_tool_script_compile/open()