Prevent cd and cl mappings from escaping

Without this, cd and cl stay mapped even when leaving netrw, which
can get a bit confusing.
More :keepalt protection

Prevent ~ too from changing the alternate file name.
If one really wants to jump quickly to the last directory without using
'proper' - and <CR>, they can still do it with <C-O> and <C-I>, but they
will loose the dirstack. (<C-O> and <C-I> are now :keepalt safe too)
Merge remote-tracking branch 'upstream/master'
Merge branch 'crumb-trail'