syscall: fix a few Linux system calls

These functions claimed to return error (an interface)
and be implemented entirely in assembly, but it's not
possible to create an interface from assembly
(at least not easily).

In reality the functions were written to return an errno uintptr
despite the Go prototype saying error.
When the errno was 0, they coincidentally filled out a nil error
by writing the 0 to the type word of the interface.
If the errno was ever non-zero, the functions would
create a non-nil error that would crash when trying to
call err.Error().

Luckily these functions (Seek, Time, Gettimeofday) pretty
much never fail, so it was all kind of working.

Found by go vet.

LGTM=bradfitz, r
R=golang-codereviews, bradfitz, r
CC=golang-codereviews
https://golang.org/cl/99320043

src/pkg/syscall/syscall_unix_test.go

@@ -300,3 +300,15 @@ func TestRlimit(t *testing.T) {
 		t.Fatalf("Setrlimit: restore failed: %#v %v", rlimit, err)
 	}
 }
+
+func TestSeekFailure(t *testing.T) {
+	_, err := syscall.Seek(-1, 0, 0)
+	if err == nil {
+		t.Fatalf("Seek(-1, 0, 0) did not fail")
+	}
+	str := err.Error() // used to crash on Linux
+	t.Logf("Seek: %v", str)
+	if str == "" {
+		t.Fatalf("Seek(-1, 0, 0) return error with empty message")
+	}
+}