options: improve handling of negations when parsing space separated values

Improve the space separated list parser to interprete "val1 ! val2"
as ("val1", "!val2") instead of ("val1", "!", "val2").

This corrects parsing of sections like ...

    config rule
        option sec_ip '! 1.1.1.0/24'

... which previously errored out with:

    Warning: Option @rule[0].src_ip has invalid value '!'

Fixes FS#806.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>