per PR 11417 and subsequent discussion with Robert Elz & others.
the length check is still enforced in skeyinit(1), but at least this
change allows older skeykeys to be used. -f is now a no-op.
Note that this is not necessarily correct for the password prompt: ideally,
libc/gen/getpass.c should be cloned into the password reading routine in
libskey, and it should be extended to take a prompt string, so that the prompt
and input are taken from /dev/tty, a la passwd(1).
Also, make usage() exit as is standard style, and delete the redundant exits.
