lib-managesieve: changed EPROTO error to EIO in ManageSieve string stream because it is apparently not known in BSD.

  - Built generic interface for alternative script sources.
  - Implemented dict script location.
  NOTE: ManageSieve will not work with this yet, nor will sieve_before/sieve_after.

This change matches a similar change in Dovecot for IMAP/POP3.
Also includes other small changes that make ManageSieve match IMAP implementation a little more closely.

ManageSieve: added support for reading quoted and literal strings as a stream. Fixes support for handing large SASL responses.
Also resolves long-standing FIXME regarding the second parameter of PUTSCRIPT: it can now be a quoted string.
Includes a few small changes in the login daemon that were done in the dovecot equivalents before.

Old name is now deprecated.
Support for the even older global_script_path name for this setting is dropped.

Setting sieve_max_redirects=0 now means that redirects are prohibited rather than unlimited.
Now there is always a limit, but it can be enormous if the administrator wants to.

Message substitutions would not always cause a proper snapshot.

This is needed for the new extprograms plugin.

Removes quite a bit of duplicated source code.

This puts the ".sieve" and ".svbin" file extension definitions in a single locatio.
IMPORTANT: this fixes a bug in the include extension that implicitly mapped script names like "name.sieve" to "name".

This is borrowed from Dovecot's lib-storage/index/maildir.

Upon upload, the include extension is more lenient towards circular includes and missing scripts as required by RFC.
The script is now also verified upon SETACTIVE when it was not active before.
However, this new SETACTIVE behavior is not optimal for the situation where the active script was updated with
PUTSCRIPT and contains an nonexistent include (ignored during upload). This can still cause runtime errors, since
an already active script is not verified again during SETACTIVE.

It returned the number of errors in stead of the number of warnings.

lib-sieve: made sure that modified messages are not stored in the source mailbox when it was opened read-only.
This is currently only relevant for the sieve-filter tool to avoid duplicating messages in the
source folder. This can happen when the messages are modified (e.g. by the editheader extension) and the
source folder is opened read-only, thus preventing deletion of the original message.

lib-sieve: store action: copy flags and keywords from input mail, also when imap4flags extension is not active.
This is currently only relevant for sieve-filter, which now properly preserves flags with this change.