__swsetup: set error flag and errno on error.

__swsetup: set error flag and errno on error.

Previously, we set errno to EBADF if the cantwrite() macro (which calls
__swsetup()) returns true for POSIX compliance.  However, we neglected
to also set the error flag, __SERR.  Rather than set the error flag in
all callers of cantwrite(), set both errno and the error flag in
__swsetup().  This matches what FreeBSD does and makes it possible
to choose a proper errno value for the second error condition in
__swsetup().  OK deraadt@