rewind: always clear error indicator.

Required by POSIX:
http://pubs.opengroup.org/onlinepubs/009695399/functions/rewind.html

Obtained from:	Apple Inc. (Libc 997.90.3)
MFC after:	1 week
ftello: return 1 when seeking offset on an append stream.

Obtained from:	Apple Inc. (Libc 997.90.3)
Phabric:	D442
MFC after:	2 weeks