Merge branch 'jk/use-wunreachable-code-for-devs'

Enable -Wunreachable-code for developer builds. * jk/use-wunreachable-code-for-devs: config.mak.dev: enable -Wunreachable-code git-compat-util: add NOT_CONSTANT macro and use it in atfork_prepare() run-command: use errno to check for sigfillset() error