FreeBSD: make adjustments for the standalone environment

In FreeBSD, there are three compile environments that are supported:
user land, the kernel and the bootloader / standalone. Adjust the
headers to compile in the standalone environment. Limit kernel-only
items from view when _STANDALONE is defined.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Warner Losh <imp@FreeBSD.org>
Closes #10998

include/os/freebsd/spl/sys/sig.h

@@ -29,6 +29,8 @@
 #ifndef _OPENSOLARIS_SYS_SIG_H_
 #define	_OPENSOLARIS_SYS_SIG_H_
 
+#ifndef _STANDALONE
+
 #include_next <sys/signal.h>
 #include <sys/param.h>
 #include <sys/lock.h>
@@ -62,4 +64,7 @@ issig(int why)
 	}
 	return (0);
 }
+
+#endif /* !_STANDALONE */
+
 #endif	/* _OPENSOLARIS_SYS_SIG_H_ */