Checks whether binary is suitable for execution.

Move part within data stack frame to separate function.

Currently, only CPU time is supported.

There is only one implementation now.

plugins: imap-filter-sieve: cmd-filter-sieve - Avoid using imap_arg_as_nstring() in cmd_filter_sieve_script_parse_value_arg()

The argument can only be a quoted string here, so imap_arg_as_string() will suffice.

This was not supposed to be accepted and led to unexpected behavior:

- FILTER SIEVE PERSONAL NIL was handled as FILTER SIEVE DELIVERY,
- FILTER SIEVE GLOBAL NIL caused an internal error.