xstat: use ephemeral ports for xstat_fs_test and scout

Instead of trying to bind to port 7101, and then retrying if
the port is in already in use, let the os find an available
port for scout and xstat_fs_test.

This fixes a bug where scout and xstat_fs_test do not call
rx_Finalize() before retrying rx_Init() with a different port
number, causing the program to crash later when more than
one copy of xstat_fs_test and/or scout are running at the same
time.

Reviewed-on: http://gerrit.openafs.org/10707
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit cda5e665ebee3da5615dba71b7a11ffff1229c6d)

Change-Id: I00c2d9e7d48e80dc1bc062a85788ad2de65709c7
Reviewed-on: http://gerrit.openafs.org/11155
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>