Fix flag option parsing bug in config file parser.

Fix flag option parsing bug in config file parser.

The old code misparses the config file if it contains a flag option
which is followed by a newline which starts with whitespace. In this
case, since newlines belong to the [[:space:]] class, the option is
regarded as an option with argumment.

Fix this by s/:space:/:blank:/. The [[:blank:]] class only contains
space and tab. which is what the code expects.