xz: Fix interaction between preset and custom filter chains.

xz: Fix interaction between preset and custom filter chains.

There was somewhat illogical behavior when --extreme was
specified and mixed with custom filter chains.

Before this commit, "xz -9 --lzma2 -e" was equivalent
to "xz --lzma2". After it is equivalent to "xz -6e"
(all earlier preset options get forgotten when a custom
filter chain is specified and the default preset is 6
to which -e is applied). I find this less illogical.

This also affects the meaning of "xz -9e --lzma2 -7".
Earlier it was equivalent to "xz -7e" (the -e specified
before a custom filter chain wasn't forgotten). Now it
is "xz -7". Note that "xz -7e" still is the same as "xz -e7".

Hopefully very few cared about this in the first place,
so pretty much no one should even notice this change.

Thanks to Conley Moorhous.