v1.3.2

xattr: use x/sys/unix, simplify
Use /proc/thread-self/attr if available
Add/use readAttr/writeAttr
isProcHandle: simplify usage, improve diagnostics
Fix [Set]EnforceMode and SecurityCheckContext
getSELinuxfs: simplify using sync.Once
findSELinuxfsMount: optimize
TestSetEnforceMode: separate and fix for non-root
Remove SelinuxfsMagic
go-selinux/SetKeyLabel: fix for RHEL7 kernels