It is sometimes useful to add a custom profile-set.
For that, e.g. `default.conf` needs to be modified.
(at least, i have not succeeded in just adding a new file)
But that change gets overridden when the package is updated,
which could be *extremely* dangerous, e.g. if said profile
changed the `volume-limit`.
By shipping an "empty" `9999-custom.conf`,
the update becomes less problematic,
because one can now use e.g. `dpkg-divert` on said file.
Refs. https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1050293
Motu M4 has four inputs (two line-in inputs, and two complete ones
with gain and XLR and whatnot), as well as four outputs (two monitor
pairs, and an unnaccounted headphone).
Sadly, like a few other interfaces, it wasn't being given an input
profile, since the matching code goes through default.conf testing
each config, and ends up selecting 'analog-surround-40', which does
not have input mapping. The inputs would fallback to 'multichannel-
input', which also doesn't have input paths.
Add input paths to all analog-surround-* mappings, and remove their
'direction=output' fields since they handle both out and in.
