sysfs: requires all methods to return syscall.Errno

This forces all syscall functions, notably filesystem, to return numeric
codes as opposed to mapping in two different areas. The result of this
change is better consolidation in call sites of `sysfs.FS`, while
further refactoring is needed to address consolidation of file errors.

Signed-off-by: Adrian Cole <adrian@tetrate.io>

internal/gojs/errno_test.go

@@ -39,6 +39,11 @@ func TestToErrno(t *testing.T) {
 			input:    syscall.EEXIST,
 			expected: ErrnoExist,
 		},
+		{
+			name:     "syscall.EFAULT",
+			input:    syscall.EFAULT,
+			expected: ErrnoFault,
+		},
 		{
 			name:     "syscall.EINTR",
 			input:    syscall.EINTR,