Testsuite: tested handling of teststuite envelope environment and fixed bugs in the envelope test in the process.

Reversed stat() and open() on two occasions to make retrieved stat information guaranteed to be valid for the opened file and added error handling for various close() system calls.

Removed various unnecessary includes of <ctype.h> and replaced yey another toupper() with its i_* equivalent.

Removed direct use of isdigit, isalpha and isalnum and replaced these with their dovecot i_* equivalents to prevent problems on some systems.

Introduced the concept of a sieve object to merge the coding of comparators, match types, address parts and other objects that might need to be represented in byte code (removes lots of code duplication).

Made error reporting cleaner by avoiding the scriptname of the main script and indicating that the printed numbers are in fact source code lines.