[bug#72337] Add /etc/subuid and /etc/subgid support

["Ludovic Courtès" <ludo@gnu.org>]

Hello,

Giacomo Leidi <goodoldpaul@autistici.org> skribis:

> This commit adds a new record type, <subid-entry> and serializers
> and deserializers for it in (gnu build accounts).  Each instance of this
> record represents one line in either /etc/subuid or /etc/subgid.  Since
> Shadow uses the same representation for both files, it should be ok if
> we do it as well.
>
> This commit adds also <subid-range>, a user facing representation of
> <subid-entry>. It is supposed to be usable directly in OS configurations.
>
> * gnu/build/accounts.scm (subid-entry): New record;
> (write-subgid): add serializer for subgids;
> (write-subuid): add serializer for subuids;
> (read-subgid): add serializer for subgids;
> (read-subuid): add serializer for subuids.
> * gnu/system/accounts.scm (subid-range): New record.
> * test/accounts.scm: Test them.
>
> Change-Id: I6b037e40e354c069bf556412bb5b626bd3ea1b2c

LGTM!