Define PS_USE_CLOBBER_ARGV on GNU/Hurd.

Define PS_USE_CLOBBER_ARGV on GNU/Hurd.

Until d2ea2d310dfdc40328aca5b6c52225de78432e01, the PS_USE_PS_STRINGS
option was used on the GNU/Hurd. As this option got removed and
PS_USE_CLOBBER_ARGV appears to work fine nowadays on the Hurd, define
this one to re-enable process title changes on this platform.

In the 14 and 15 branches, the existing test for __hurd__ (added 25
years ago by commit 209aa77d, removed in 16 by the above commit) is left
unchanged for now as it was activating slightly different code paths and
would need investigation by a Hurd user.

Author: Michael Banck <mbanck@debian.org>
Discussion: https://postgr.es/m/CA%2BhUKGJMNGUAqf27WbckYFrM-Mavy0RKJvocfJU%3DJ2XcAZyv%2Bw%40mail.gmail.com
Backpatch-through: 16