exec: Convert security_bprm_set_creds into security_bprm_repopulate_creds

Rename bprm->cap_elevated to bprm->active_secureexec and initialize it
in prepare_binprm instead of in cap_bprm_set_creds.  Initializing
bprm->active_secureexec in prepare_binprm allows multiple
implementations of security_bprm_repopulate_creds to play nicely with
each other.

Rename security_bprm_set_creds to security_bprm_reopulate_creds to
emphasize that this path recomputes part of bprm->cred.  This
recomputation avoids the time of check vs time of use problems that
are inherent in unix #! interpreters.

In short two renames and a move in the location of initializing
bprm->active_secureexec.

Link: https://lkml.kernel.org/r/87o8qkzrxp.fsf_-_@x220.int.ebiederm.org
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

include/linux/binfmts.h

@@ -27,10 +27,10 @@ struct linux_binprm {
 	unsigned long argmin; /* rlimit marker for copy_strings() */
 	unsigned int
 		/*
-		 * True if most recent call to cap_bprm_set_creds
+		 * True if most recent call to security_bprm_set_creds
 		 * resulted in elevated privileges.
 		 */
-		cap_elevated:1,
+		active_secureexec:1,
 		/*
 		 * Set by bprm_creds_for_exec hook to indicate a
 		 * privilege-gaining exec has happened. Used to set