postfix.inc: do not hardcode userid and groupid

It hardcoded to assign vmail's userid and groupid with 5000,
which caused the following useradd/groupadd is increased from
5000 at do_root time. Such as 'tracing' group id is 5001
(tracing group is added in lttng-tools).

It is better to allocate it dynamically.

BTW, you may google to find the hardcoded 5000 in following site:
https://help.ubuntu.com/community/PostfixVirtualMailBoxClamSmtpHowto



In its main.cf, it also hardcoded virtual_uid_maps and virtual_gid_maps.
...
virtual_uid_maps = static:5000
virtual_gid_maps = static:5000
..

But in meta-oe's main.cf_2.0, it assigned virtual_uid_maps and
virtual_gid_maps with a search.
...
virtual_uid_maps = static:`grep vmail /etc/passwd | cut -d ":" -f 3`
virtual_gid_maps = static:`grep vmail /etc/passwd | cut -d ":" -f 4`
...

So please feel free to forget 5000.

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>