Don't resolve symlinks with cd

This switches fish to a "virtual" PWD, where it no longer uses getcwd to
discover its PWD but instead synthesizes it based on normalizing cd against
the $PWD variable.

Both pwd and $PWD contain the virtual path. pwd is taught about -P to
return the physical path, and -L the logical path (which is the default).

Fixes #3350

tests/cd.err

@@ -0,0 +1,3 @@
+
+####################
+# cd symlink non-resolution