mwan3: write `mwan3 use` error/debug messages to stderr instead of stdout

mwan3: write `mwan3 use` error/debug messages to stderr instead of stdout

This redirects the debug output to stderr, allowing `mwan3 use` to be used in
scripts without polluting stdout.

Before:

mwan3 use wan curl -fsSL https://ifconfig.co/json | jq -er '.country_iso'
jq: parse error: Invalid numeric literal at line 1, column 8
curl: (23) Failure writing output to destination, passed 389 returned 0

After:

mwan3 use wan curl -fsSL https://ifconfig.co/json | jq -er '.country_iso'
Running 'curl -fsSL https://ifconfig.co/json' with DEVICE=eth2 SRCIP=192.168.0.1 FWMARK=0x3f00 FAMILY=ipv4
DE

Signed-off-by: Philipp Schmitt <philipp@schmitt.co>