Fixed a few printf format problems in the process.
Updated test suite to account for the new number width.

It couldn't handle a missing parameter (which also means missing arguments of the test itself in most cases).
This is fixed by using the sieve_validate_tag_parameter() utility function.
Testsuite is also extended.

This allows extension of test commands that evaluate the message somehow.
Currently this is implemented only for the message header.
Extensions can thereby influence what headers are evaluated and how.

Based on patch by Timo Sirainen.

Improved error message for unknown Sieve extension to account for core commands included as an extension.

Enforced limits on string length and handled the finite nature of integers for number parsing and the construction of the AST.