torture: Use mktemp instead of guessing at unique names

This commit drags the rcutorture scripting kicking and screaming into the
twenty-first century by making use of the BSD-derived mktemp command to
create temporary files and directories. In happy contrast to many of its
ill-behaved predecessors, mktemp seems to actually work reasonably reliably!

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

tools/.../rcutorture: Convert to SPDX license identifier

Replace the license boiler plate with a SPDX license identifier.
While in the area, update an email address and add copyright notices.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

torture: Provide TMPDIR environment variable to specify tmpdir

Both rcutorture and locktorture currently place temporary files in /tmp,
in keeping with decades-long tradition. However, sometimes it is useful
to specify an alternative temporary directory, for example, for space
or performance reasons. This commit therefore causes the torture-test
scripting to use the path specified in the TMPDIR environment variable,
or to fall back to traditional /tmp if this variable is not set.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

torture: Add --kconfig argument to kvm.sh

Currently, testing a variant of an existing scenario requires editing
that scenario's file or creating a new scenario file. This is messy
and error prone with respect to changes to scenarios.

This commit therefore adds a --kconfig argument to kvm.sh, so that
'--kconfig "CONFIG_RCU_TRACE=y CONFIG_RCU_EQS_DEBUG=n" will override those
two Kconfig options. In addition, there is now clear precedence:
the config fragment overrides CFcommon, and the --kconfig argument
overrides both.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>