fw4: ensure to capitalize weekday names

fw4: ensure to capitalize weekday names

Despite what's being documented in the nftables wiki, nftables does not
actually accept lowercased or abbreviated weekday names, currently failing
with a "Error: Could not parse Day of week of packet reception" error for
expressions such as `meta day { "sunday", "wed" }`.

Work around the problem by explicitly emitting capitalized weekday names
while a more thorough nftables-side solution is being sent upstream.

Ref: https://forum.openwrt.org/t/firewall-time-restrictions-parental-controls-problems/123964
Ref: https://wiki.nftables.org/wiki-nftables/index.php/Matching_packet_metainformation#Matching_by_time
Signed-off-by: Jo-Philipp Wich <jo@mein.io>