lib-sieve: sieve-extensions - Notify caller of sieve_extensions_set_string() about extension load failures

Was handled in each extension before, which is useless.

lib-sieve: Changed sieve_extension_override() to use hash_table_update() rather than hash_table_insert().

Due to a Dovecot change this would now cause an assertion failure.

This would uselessly allocate a new entry with the same content.

These are not available under that name anymore and when the replacing extension has the same bare name, this would have led to duplicate capabilities.

It allows sending feedback report (RFC 5965) messages.

The extensions listed in this setting do not need to be enabled explicitly using the Sieve require command.
This behavior voilates the standard, but is necessary for compatibility with some existing implementations of Sieve.
Do not use this setting unless you really need to!

This adds the vnd.dovecot.default-mailbox and vnd.dovecot.username environment items.
It creates the 'env' variables namespace through which all environment items can be accessed directly.
This extension is yet to be documented properly.
More environment items are under consideration.