torture: Remove use of "eval" in torture.sh

The bash "eval" command enables Bobby Tables attacks, which might not
be a concern in torture testing by themselves, but one could imagine
these combined with a cut-and-paste attack.  This commit therefore gets
rid of them.  This comes at a price in terms of bash quoting not working
nicely, so the "--bootargs" argument lists are now passed to torture_one
via a bash-variable side channel.  This might be a bit ugly, but it will
also allow torture.sh to grow its own --bootargs parameter.

While in the area, add proper header comments for the bash functions.

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