Since the script storage code is widely engrained in the Pigeonhole sources,
this is a very large commit that cannot be reduced further into smaller
preparatory bits.

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

For now, these yield the same result.

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

plugins: imapsieve: imap-sieve-settings - Properly check imapsieve_url setting for Sieve URL validity

It doesn't belong there.

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

plugins: doveadm-sieve: doveadm-sieve-sync - Refactor error handling in sieve_attribute_set_active()

Was handled in each extension before, which is useless.

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