ssh: add save-mutex-profile

[wadey]

This change lets you call mutex-profile-fraction to set the mutex profile sampling rate, and then save-mutex-profile to save the profile to a file on disk.

This can be helpful when profiling to check for mutex contention issues.

• https://pkg.go.dev/runtime#SetMutexProfileFraction
• https://pkg.go.dev/runtime/pprof#Profile