Bug#952864: [live-build] true|false are confusing values for --debian-installer

[jnqnfe@gmail.com]

Package: live-build
Version: 1:20191221
Owner: jnqnfe@gmail.com

the --debian-installer option accepts values of
true|cdrom|netinst|netboot|businesscard|live|false. Having 'true' and
'false' here is very confusing. the set of values needs replacing with
cdrom|netinst|netboot|businesscard|live|none.

note that `true` is being treated as an alias for `netinst`. in the new
set of values `none` would obviously be equivalent to the current
`false`.

a patch for this was previously submitted in a large patch set that was
never merged. I looked at it the other day to refresh my mind, though
do not recall the bug number and can't be bothered to dig it up, and I
remember a review comment specifically referring to my patch for this,
not liking the fact that the patch had no backwards compatibility. such
backwards compatibility has been reworked into this resubmission.

patch to be submitted via salsa shortly (will amend commit with bug
number first)