hurd compatibility changes

hurd compatibility changes

Hurd does not define PIPE_BUF, so lib/tevent/testsuite.c fails to compile
(yes, this file is used as part of *samba* testsuite, not tevent testsuite).
Define it to a safe minimal value like 512 bytes.

Hurd does not provide SA_NOCLDWAIT define, so lib/util/tests/tfork.c does
not compile. This is only needed during testing to omit zombie process
generation, which has only cosmetic effect.  Define it to be 0.

Based on prior work and ideas by Samuel Thibault.

Gbp-Pq: Name hurd-compat.patch