tree-wide: add path_simplify_alloc() and use it

tree-wide: add path_simplify_alloc() and use it

path_simplify_full()/path_simplify() are changed to allow a NULL path, for
which a NULL is returned. Generally, callers have already asserted before that
the argument is nonnull. This way path_simplify_full()/path_simplify() and
path_simplify_alloc() behave consistently.

In sd-device.c, logging in device_set_syspath() is intentionally dropped: other
branches don't log.

In mount-tool.c, logging in parse_argv() is changed to log the user-specified
value, not the simplified string. In an error message, we should show the
actual argument we got, not some transformed version.