net/acme-acmesh: prevent concurrent execution of acme.sh instances

net/acme-acmesh: prevent concurrent execution of acme.sh instances

After switching ACME to being run through procd, individual ACME
instances are now executed in parallel instead of sequentially. This
causes acme.sh to become confused in certain cases. Petr reported that
certificate files ended up with the wrong contents after updating to
OpenWrt 25.12.

Fix this by wrapping the acme.sh execution in a global `flock` keyed on
a file in the state directory. The procd instances will still run in
parallel, but they'll all serialise on the same lock, so only one
acme.sh instance will run at a time.

Fixes: 76b676e4ebbb ("acme.sh: move to procd to ensure logging gets to syslog")
Reported-by: Petr Novák <redacted>
Signed-off-by: Toke Høiland-Jørgensen <redacted>