basic/path-util: make path_simplify() skip leading '/..'

basic/path-util: make path_simplify() skip leading '/..'

I.e., /.. becomes /, /../foo becomes /foo, /../../bar becomes /bar, etc. We can
do this unconditionally, without access to the file system, because the parent
of the root directory always resolves to. /.. in other places is handled as
before, because resolving it properly would require access to the file system
which we don't want to do in path_simplify().