This patch makes it possible to configure the CPUs on a Linux
system other than the one on which the chcpu command is called.
To achieve this users can simply define the root directory of
the sys/ directory with the --sysroot command line option. It
is also beneficial for regression tests, as these make use of
sysfs tar archive dumps.

To properly implement this new feature, the syspath initialization
had to be deferred until after all option arguments, especially
--sysroot, have been parsed, which simplified the path access check.

Along the way a small refactoring of the enable,disable,configure &
deconfigure option parsing had been done, more precisely the CPU list
option argument is now saved in a variable 'cpu_list_arg' to defer
the CPU list validity check until after all option arguments have
been parsed to simplify the logic. Lastly, getopt(3)'s 'optarg' variable
is used instead of argv[optind-1] to store the options argument, which
is more idiomatic and readable.

Signed-off-by: Christian Goeschel Ndjomouo <cgoesc2@wgu.edu>

Previously, cpu_enable() and cpu_configure() compared the number
of fails with ctx->maxcpus and if equal it returned -1, which
culminated over to an exit code of 1 (failure). Otherwise, if
there were fails but less than ctx->maxcpus the fucntions return
code 1 would result in an exit code 64 indicating partial success.

Since ctx->maxcpus describes the maximum number of CPUs supported
by the kernel, in practice, this failure mode isn't really useful
as it means that a user would need to fail the configuration of
every possible CPU. This is why chcpu(8) almost always returns 64.

It is better to store the actual amount of requested CPUs and then
compare the amount of fails to it and provide more meaningful exit
codes.

Signed-off-by: Christian Goeschel Ndjomouo <cgoesc2@wgu.edu>