SELinux user for user's login.
* NEWS, src/usermod.c, man/usermod.8.xml: Likewise.
* libmisc/system.c, libmisc/Makefile.am, lib/prototypes.h: Added
safe_system(). Used to run semanage.
* lib/prototypes.h, libmisc/copydir.c: Make a
selinux_file_context() an extern function.
* libmisc/copydir.c: Reset SELinux to create files with default
contexts at the end of copy_tree().
* NEWS, src/userdel.c: Delete the SELinux user mapping for user's
login.
