main: print "try --help" on invalid option
main: print "try --help" on invalid option

This is what ls does:

  $ ls --aaaaaaaaaaaaaaa
  ls: unrecognized option '--aaaaaaaaaaaaaaa'
  Try 'ls --help' for more information.

and we should as well.

Fixes https://github.com/rfjakob/earlyoom/issues/58

rfjakob/earlyoom,rfjakob/earlyoom,rfjakob/earlyoom,rfjakob/earlyoom