main: don't tell the user to choose a password when -extpass is used
main: don't tell the user to choose a password when -extpass is used

Instead, print this:

  Using password provided via -extpass.

rfjakob/gocryptfs,rfjakob/gocryptfs,davesteele/gocryptfs,rfjakob/gocryptfs,davesteele/gocryptfs,davesteele/gocryptfs,rfjakob/gocryptfs