parser: allow quoted string in flowtable_expr_member

parser: allow quoted string in flowtable_expr_member

Devices with interface names starting with a digit can not be configured
in flowtables. Trying to do so throws the following error:

Error: syntax error, unexpected number, expecting comma or '}'
devices = { eth0, 6in4-wan6 };

This is however a perfectly valid interface name. Solve the issue by
allowing the use of quoted strings.

Suggested-by: Jo-Philipp Wich <jo@mein.io>
Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>