support/scripts/mkusers: allow option for system uid/gid

Some software decides based on uid/gid whether a user is a system or
normal (human) user, with different behaviour for those flavors (example
journald [2]).

So adding logic to create system-users is necessary, we take the now
common ranges from [1].

This extends the mkusers script to allow -2 for uid/gid, this argument
will take an identifier from the user range. All identifiers used up to
now should have been from the system range, so -1 is now interpreted as
a system user/group.

Note that after this commit, all the UIDs and GIDs that are created
automatically (with -1) will change. That means if there is peristent
data on an existing system that was created by such an automatic user,
it will suddenly belong to a different user. However, this could already
happen before: if a USERS line is added to a package, then other UIDs
may change as well.

Add system/user ranges as variables, and the argument for user/system
uid variable as well. Thus some magic constants could be removed, some
further occurences of -1 were replaced with equivalent logic. For
consistency, the existing MIN/MAX_UID/GID variables are renamed to
FIRST/LAST_USER_UID/GID.

Update the documentation with the new automatic ranges.

[1] - https://systemd.io/UIDS-GIDS/
[2] - https://www.freedesktop.org/software/systemd/man/journald.conf.html

Signed-off-by: Norbert Lange <nolange79@gmail.com>
[Arnout: use -1 for system users; refactor the changes a bit]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

docs/manual/makeusers-syntax.txt

@@ -20,13 +20,16 @@ Where:
   It can not be +root+, and must be unique. If set to +-+, then just a
   group will be created.
 - +uid+ is the desired UID for the user. It must be unique, and not
-  +0+. If set to +-1+, then a unique UID will be computed by Buildroot
-  in the range [1000...1999]
+  +0+. If set to +-1+ or +-2+, then a unique UID will be computed by
+  Buildroot, with +-1+ denoting a system UID from [100...999] and +-2+
+  denoting a user UID from [1000...1999]. 
 - +group+ is the desired name for the user's main group. It can not
   be +root+. If the group does not exist, it will be created.
 - +gid+ is the desired GID for the user's main group. It must be unique,
-  and not +0+. If set to +-1+, and the group does not already exist, then
-  a unique GID will be computed by Buildroot in the range [1000..1999]
+  and not +0+. If set to +-1+ or +-2+, and the group does not already
+  exist, then a unique GID will be computed by Buildroot, with +-1+
+  denoting a system GID from [100...999] and +-2+ denoting a user GID
+  from [1000...1999]. 
 - +password+ is the crypt(3)-encoded password. If prefixed with +!+,
   then login is disabled. If prefixed with +=+, then it is interpreted
   as clear-text, and will be crypt-encoded (using MD5). If prefixed with