allow all iX86 strings for lxc.arch

This change accepts all the same strings for lxc.arch that setarch(8) does.

Note that we continue to parse plain x86 as PER_LINUX32 so as not to break
existing lxc configuration files.

Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
output lxc.arch as i686 for PER_LINUX32

When outputing the lxc.arch setting, use i686 instead of x86 since the
later is not a valid input to setarch, nor will the kernel output
UTS_MACHINE as x86. The kernel sets utsname.machine to i[3456]86, which
all map to PER_LINUX32.

Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>