The action name can be determined from the sieve_action_name field that existed
before.

Before it always used the name of the action definition, which is confusing e.g.
for the keep and fileinto actions, which both map to the same "store" action
definition.

No duplicate check function was implemented, so that the discard action would be duplicated each time it is invoked.
This only affects the action limits (each discard invocation is counted as another new action).
The result of the script execution would be identical.

This way, a duplicate is only marked when the script execution succeeds, as required in the specification.

This change is bigger than I would have liked, so this needs to be tested more.

This is needed for the new extprograms plugin.

lib-sieve: optimized compilation of tests that yield constant results (i.e. known at compile tme), such as true and false.
If the result of a test is known at compile time, it is optimized away. If an if-command depends on an entirely constant test, it is optimized away as well,
causing only the 'true' sub-block to be compiled.

- 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.