fix return code for pipe(2) syscall

[Garrett Wollman]

In article <20060127093602.GO83922 at cell.sick.ru> you write:

>Yes, according to SUSv3 the only errors from pipe(2) are ENFILE
>and EMFILE.

POSIX does not define an exhaustive enumeration of error conditions.
*Any* error return is permissible, provided only that *for those
conditions noted in the ERRORS section* the code identified for that
condition is returned.  It is perfectly permissible for every system
call to fail with [ENOTADUCK] unless the first five bytes of the
caller's address space contain the word "quack".

-GAWollman