Add GNU glibc compatible secure_getenv

Add mostly glibc and msl compatible secure_getenv. Return NULL if
issetugid() indicates the process is tainted, otherwise getenv(x). The
rational behind this is the fact that many Linux applications use this
function instead of getenv() as it's widely consider a, "best
practice".

Reviewed by: imp, mjg (feedback)
Pull Request: https://github.com/freebsd/freebsd-src/pull/686
Signed-off-by: Lucy Marsh <seafork@disroot.org>