[patch] Support modern network utilities for finding free ports for tests

Hello,

GNU/Linux distributions are deprecating some legacy IP tools [1], moving
them to a -deprecated package: in this case netstat. Please review the
attached patch adding compatible support for it's functional successor
ss(8).

[[[
Support modern network utilities for finding free ports for tests

* subversion/tests/cmdline/davautocheck.sh:
  Check for presence of ss, netstat and fail if neither is found.
  Prefer ss to find a free port, fall back to netstat.
* subversion/tests/cmdline/svnserveautocheck.sh:
  Same, and augment $PATH to include /usr/{,local/}sbin.

]]]

[1] https://build.opensuse.org/request/show/308848

With kind regards,
Andreas Stieger