Before, only storage errors had the "error" field set.

There is no reason to forbid an empty Sieve script.

Clang -Wvargargs complained about passing a bool argument to va_start().

Nobody does that, but we support it.

Doveadm-sieve plugin may be broken at this point; not tested so far.

Also contains a few other small fixes by Timo.

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

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.

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.

- Added sieve_global_extensions setting.

Fixed Sieve script name checking to properly handle length issues and added 0x00ff as invalid character.

- Changed operand read functions to prove exec status return codes in stead of boolean.
- Restructured optional-operand, stringlist and string-match APIs to provide execution status (error code) upon failure.
- Corruption errors are only produced when the binary is actually corrupt. Normal failures should never cause a recompile attempt anymore.
- Simplified optional-operand API.
- Cleaned up test command implementations that use the string-match API.