Add symlink support to mockFs

Add support for specifying symlinks in mock filesystems to prepare
for glob symlink tests.

This patch leaves incorrect behavior by not walking symlinks in
mockFs.ListDirsRecursive, but it matches what osFs does.

Test: fs_test.go
Change-Id: If87a83c00f21e14696faf890b7b09e88b18e95b9