Make `G_URI_FLAGS_PARSE_RELAXED` available instead, for the
implementations which need to handle user-provided or incorrect URIs.
The default should nudge people towards being compliant with RFC 3986.

This required also adding a new `G_URI_PARAMS_PARSE_RELAXED` flag, as
previously parsing param strings *always* used relaxed mode and there
was no way to control it. Now it defaults to using strict mode, and the
new flag allows for relaxed mode to be enabled if needed.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Fixes: #2149