MFC: r269806,r269809,r269811,r269810

r269806:
Fix too long (seed length >12 chars) challenge handling.
1) " ext" length should be included into OPIE_CHALLENGE_MAX (as all places
of opie code expects that).
2) Overflow check in challenge.c is off by 1 even with corrected
OPIE_CHALLENGE_MAX
3) When fallback to randomchallenge() happens and rval is 0 (i.e.
challenge is too long), its value should be set to error state too.

To demonstrate the bug, run opiepasswd with valid seed:
opiepasswd -s 1234567890123456
and notice that it falls back to randomchallenge() (i.e. no
1234567890123456 in the prompt).

r269809:
When sha1 support was added, they forget to increase OPIE_HASHNAME_MAX

r269811:
Last '/' for program name, not first one.

r269810:
Link otp-sha1 to match real challenge prompt, not otp-sha.

PR:     191511
Submitted by: mitsururike@gmail.com (partially, PR 269806)