default flag value can fail to pass through on reload

[crawshaw]

Added a flag to a process, so on -littleboss=reload it tried and failed to use the default value:

panic: littleboss: child listener flag "myaddr" value does not include fd: ":8465"

goroutine 1 [running]:
crawshaw.io/littleboss.(*Littleboss).child(0xc0000e4dc0, 0x9a2848)
        /Users/crawshaw/go/src/crawshaw.io/littleboss/littleboss.go:879 +0xb1e
crawshaw.io/littleboss.(*Littleboss).Run(0xc0000e4dc0, 0x9a2848)
        /Users/crawshaw/go/src/crawshaw.io/littleboss/littleboss.go:247 +0x961
main.main()
        /Users/crawshaw/go/src/spilled.ink/smsmtpd/main.go:64 +0x2bc