Remove XXX comments complaining that write(2) on a read-only descriptor
returns EBADF. That errno is correct and is mandated by POSIX. It also
goes back to revision 1.1 of our CVS history (i.e. 4.4BSD).

The _fget() function should probably also be upated as it currently returns
EINVAL in that case rather than EBADF. (It does return EBADF for reads
on a write-only descriptor without any XXX comments oddly enough.)

Discussed with: scottl, grog, mjacob, bde