rundir.h: fix multi-include header guard

daemon.h defines __LIBDAEMON_DAEMON_H, so rundir.h should not use
this same value to guard against multiple includes.  Ultimately,
it would be best to user #pramga once, so long as all the platforms
libdaemon compiles on will accept that.
Merge pull request #1 from reubenhwk/rundir-fix

rundir.h: fix multi-include header guard