Declare time(not3) instead of depending on namespace pollution 3 layers

deep in <stand.h> to eventually include <time.h> to declare the user
version.

This is not quite the right place to declare it, but <stand.h> would
be worse because time() is very MD so it isn't in libstand.

Many places in the boot sources still get the user version using only
1 layer of pollution (#include <sys/time.h>.  Some pollute themselves
directly (#include <time.h>).  But the boot Makefiles are too broken
to enable warnings for redeclarations.

sys/boot/common/bootstrap.h

@@ -285,3 +285,5 @@ extern struct arch_switch archsw;
 void	delay(int delay);
 
 void	dev_cleanup(void);
+
+time_t	time(time_t *tloc);