libselinux: state setexecfilecon(3) sets errno on failure

The other functions (getexeccon(3) and setexeccon(3)) from the man page
also set errno on failure similar to the getcon(3) function family.

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
Acked-by: James Carter <jwcart2@gmail.com>

libselinux/man/man3/getexeccon.3

@@ -89,7 +89,9 @@ then executes the specified filename with the provided argument and
 environment arrays.
 .
 .SH "RETURN VALUE"
-On error \-1 is returned.
+On failure, \-1 is returned and
+.I errno
+is  set appropriately.
 
 On success
 .BR getexeccon (),