Sequential exections would override a previously flagged delivery, which is wrong.
This caused the sieve_discard feature to trigger for messages that were actually delivered somewhere by an earlier script in the sequence.

The simplest case in which all normal scripts maintain the (implicit) keep action triggered an assert failure.
Fixed by restructuring the script execution loop to make it easy to understand.
Now handles each case explicitly.

lib-sieve: vnd.dovecot.report extension: Added support for configuring the "From:" address used in the report.

program_client_remote_create() is now called program_client_unix_create().

Assert failure could occur if the input buffer was full.
Also, a small chance existed that not all data would be saved.

LDA Sieve plugin: Implemented support for a "discard script" that is run when the message is going to be discarded.

This adds a new "sieve_discard" setting that points to the location of the "discard script".
Once the normal scripts finish executing and the result is that the message will be discarded without being delivered anywhere, the "discard script" is executed.
The discard script can prevent dropping the message, by executing alternative actions.
If the discard script does nothing, the message is still discarded.

The "discard script" is run when the normal script sequence ends and the message is going to be discarded.

Removed special code path for single script.
This is a remnant from times when the multiscript feature was still experimental.

A "vnd.dovecot.config.<identifier>" environment item maps to the content of the matching sieve_env_<identifier> plugin setting.

lib-sieve: environment extension: Added support for wildcard environment items that are identified by a name prefix.

lib-sieve: variables extension: Substitution of an invalid namespace canceled the substitution, rather than substituting the empty string.

lib-sieve: variables extension: Increased the maximum number of namespace elements in a variable name.

Bug was exposed by previous change. Normally, this would not surface.

There is no reason to forbid an empty Sieve script.

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

Found with clang -Wstrict-bool.

Found with clang -Wstrict-bool.

Addresses a FIXME.
Caused a warning with clang -Wstrict-bool.

Found with clang -Wstrict-bool.

Found with clang -Wstrict-bool.

Found with clang -Wstrict-bool.

lib-sieve: file storage: The try_hardlink parameter of file_copy() is of type bool, rather than int.

Found with clang -Wstrict-bool.

Found with clang -Wstrict-bool.

lib-sieve: script: The return type of binary_dump_metadata() is supposed to be bool rather than int.

Found with clang -Wstrict-bool.

Found with clang -Wstrict-bool.

Found with clang -Wstrict-bool.

Found with clang -Wstrict-bool.

Found with clang -Wstrict-bool.

Mixed up bool and int return types.
Found with clang -Wstrict-bool.

Found with clang -Wstrict-bool.

Mixed up bool and int return types.
Found with clang -Wstrict-bool.

Mixed up bool and int return types.
Found with clang -Wstrict-bool.

Found with clang -Wstrict-bool.