Repair dirname and basename

Repair dirname and basename

basename and dirname must be able to handle paths ending in zero or more
slashes. Concatenation of dir- and basename will otherwise go wrong in
edge cases.

previously:
dirname(foo/bar/) -> foo/bar
basename(foo/bar/) -> bar
=> foo/bar/bar

now:
dirname(foo/bar///) -> foo
basename(foo/bar///) -> bar
=> foo/bar

Signed-off-by: Tobias Ulmer <tu@emlix.com>