Resulting fseek() offset must fit in long, required by POSIX (pointed by bde),
so add LONG_MAX and final tests for it.
rewind.c:
1) add missing __sinit() as in fseek() it pretends to be.
2) use clearerr_unlocked() since we already lock stream before _fseeko()
3) don't zero errno at the end, it explicitely required by POSIX as the
only one method to test rewind() error condition.
4) don't clearerr() if error happens in _fseeko()
"[EINVAL] ... The resulting file-position indicator would be set to a
negative value."
Moreover, in real life negative seek in stdio cause EOF indicator cleared
and not set again forever even if EOF returned.
2) Catch few possible off_t overflows.
Reviewed by: arch discussion
compiling, since <stdio.h> correctly doesn't declare off_t although
the pseudo-prototypes for the new fseeko() and ftello() functions
use it. Handle this like the corresponding problem for va_list
versus the vprintf() family.
Fixed some English errors.
