Description of issue or feature request:
#1008 removes ambiguity in interfaces that take TARGETPATH and/or PATHPATTERN arguments, by only accepting paths and path patterns that use "/" as directory separator and don't start with "/".

To make the interface even stronger, we should only accept paths and path patterns that completely adhere to the recommended format.

Note that, although the specification only recommends a format, we agreed that an implementation can be more assertive than the spec (see spec#63 and spec#67).

Current behavior:

• Functions that take TARGETPATH and/or PATHPATTERN arguments only accept strings that use "/" as directory separator and don't start with "/".

Expected behavior:

• Functions that take TARGETPATH arguments only accept path-relative-scheme-less-URL strings.

• Functions that take PATHPATTERN arguments only accept path-relative-scheme-less-URL strings that may include shell-style glob characters. I suggest to align this with whatever the client updater supports.