selinux: de-brand SELinux

selinux: de-brand SELinux

Change "NSA SELinux" to just "SELinux" in Kconfig help text and
comments. While NSA was the original primary developer and continues to
help maintain SELinux, SELinux has long since transitioned to a wide
community of developers and maintainers. SELinux has been part of the
mainline Linux kernel for nearly 20 years now [1] and has received
contributions from many individuals and organizations.

[1] https://lore.kernel.org/lkml/Pine.LNX.4.44.0308082228470.1852-100000@home.osdl.org/

Signed-off-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>