rtld: Add glibc.rtld.enable_secure tunable.

Add a tunable for setting __libc_enable_secure to 1.  Do not set
__libc_enable_secure to 0 if the tunable is set to 0.  Ignore all
tunables if glib.rtld.enable_secure is set.  One use-case for this
addition is to enable testing code paths that depend on
__libc_enable_secure being set without the need to use setxid binaries.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>