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

Was handled in each extension before, which is useless.

Dovecot core doesn't have copyright notices in header files.

perl -i -pe 's/unsigned int ([^,:;]+):1;/bool $1:1;/' **/*.[ch]

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!

Those version numbers are now also stored in the binary, making sure it is recompiled
when a new version of an extension is used. This avoids updating the binary version
for minor changes to extensions and also makes this work for plugins.

- Added sieve_global_extensions setting.

Made vnd.dovecot.debug extension available to the LDA plugin instead of only the command line tools.

Imap4flags: added support for obsolete imapflags extension for backwards compatibility with CMUSieve.