Found with clang -Wstrict-bool.

lib-sieve: code: Added assert to prevent emitting invalid byte code when extension definitions are wrong.

Needed for test suite.

Removes quite a bit of duplicated source code.

- 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.

Be explicit about signedness of data in binary code representation. Some architectures, like ARM, differ in this respect.

Exported variable string operand into the Sieve engine itself as 'catenated string' (for similar use in other extensions like the testsuite).

Previously the final value of internal variable was for every store action that didn't
specify a :flags argument explicitly. This results in out-of order assignment/removal
of flags, e.g. also the flags assigned keep actions that were executed before the
addflag/setflag command were modified.