Clarified the wording of the RETURN VALUES section once again. Fseek

returns -1 on error, the others return non-zero.
The wording is still awkward, fgetpos & fsetpos should probably get a
separate manual page.  But this should be good enough for the release.

lib/libc/stdio/fseek.3

@@ -138,7 +138,11 @@ return 0,
 and
 .Fn ftell
 returns the current offset.
-Otherwise, a nonzero value is returned and the global variable errno is 
+Otherwise, 
+.Fn fseek
+returns \-1 and 
+the others
+return a nonzero value and the global variable errno is 
 set to indicate the error.
 .Sh ERRORS
 .Bl -tag -width [EINVAL]