Add new generic functions to find the next user or group ID available:

Add new generic functions to find the next user or group ID available:
find_new_uid() and find_new_gid(). They work the same way as the functions
with the same name of useradd or groupadd, except that they check in the
local database to make sure an ID was not reserved in an uncommitted
change (this is needed to be used in newusers), they report a status
instead of calling exit(), and they can receive a preferred ID. They
should later support system IDs. This should be a little bit slower, but
not too much (if the database is not open the checks against the local
database will exit immediately, and if it is already open, all the checks
will be done regarding the data in memory).