Fix data race in the signal package logger during shutdown (issue #6137).

The mainInterruptHandler goroutine reads the package-level log variable while UseLogger() may be called from the main goroutine during startup to replace the logger with a properly configured one. This concurrent access without synchronization causes a data race when lnd is built with the -race flag.

The Problem

When lnd starts:

1. signal.Intercept() spawns mainInterruptHandler goroutine which reads log to handle shutdown signals
2. Main goroutine calls SetupLoggers() → signal.UseLogger() which writes to log
3. Both goroutines access the same log variable without synchronization

This race was reported in issue #6137 and can be reproduced by:

• Building lnd with -race flag
• Running lnd in simnet with btcd backend
• Sending SIGINT during early startup

The Solution

Add a sync.RWMutex to protect concurrent access to the log variable:

• UseLogger() acquires write lock when replacing the logger
• New getLogger() helper acquires read lock when accessing the logger
• All direct log accesses in signal.go replaced with getLogger() calls

This ensures thread-safe access while maintaining good performance (read-heavy workload benefits from RWMutex).

Fixes #6137

Steps to Test

1. Build lnd with race detector:

   go build -race -o lnd-race ./cmd/lnd

2. Start btcd in simnet:

   btcd --simnet --rpcuser=devuser --rpcpass=devpass \
     --rpclisten=127.0.0.1:18556 --listen=127.0.0.1:18555 \
     --rpccert=/tmp/btcd/rpc.cert --rpckey=/tmp/btcd/rpc.key \
     --txindex

3. Start lnd with race detector:

   ./lnd-race --bitcoin.simnet --bitcoin.node=btcd \
     --btcd.rpcuser=devuser --btcd.rpcpass=devpass \
     --btcd.rpchost=127.0.0.1:18556 \
     --btcd.rpccert=/tmp/btcd/rpc.cert \
     --noseedbackup

4. Send SIGINT during startup:

   kill -SIGINT <lnd-pid>

5. Verify no data race warnings appear in logs related to signal/signal.go or signal/log.go

Before the fix: Data race detected between mainInterruptHandler (read) and UseLogger (write)

After the fix: No data race warnings

Pull Request Checklist

Testing

• Your PR passes all CI checks.
• Tests covering the positive and negative (error paths) are included.
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• Any new lncli commands have appropriate tags in the comments for the rpc in the proto file.
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