Description of problem:
Default setting of first_valid_uid conflicts with system users: The default
as per /etc/dovecot/conf.d/10-mail.conf is 500 while real system users are
also above UID 500, because the regular users at RHEL start at UID 1000. So
this is leading to the following errors by default:

$ doveadm purge -A
doveadm(polkitd): Error: user polkitd: Initialization failed: Namespace '': mkdir(/home/polkitd/mailboxes) failed: Permission denied (euid=999(polkitd) egid=997(polkitd) missing +w perm: /home, dir owned by 0:0 mode=0755)
doveadm(polkitd): Error: User init failed
doveadm(dovenull): Error: user dovenull: Initialization failed: Namespace '': mkdir(/home/dovenull/mailboxes) failed: Permission denied (euid=998(dovenull) egid=996(dovenull) missing +w perm: /home, dir owned by 0:0 mode=0755)
doveadm(dovenull): Error: User init failed
doveadm(unbound): Error: user unbound: Initialization failed: Namespace '': mkdir(/home/unbound/mailboxes) failed: Permission denied (euid=997(unbound) egid=995(unbound) missing +w perm: /home, dir owned by 0:0 mode=0755)
doveadm(unbound): Error: User init failed
doveadm(saslauth): Error: user saslauth: Initialization failed: Namespace '': mkdir(/home/saslauth/mailboxes) failed: Permission denied (euid=996(saslauth) egid=76(saslauth) missing +w perm: /home, dir owned by 0:0 mode=0755)
doveadm(saslauth): Error: User init failed
doveadm(nfsnobody): Error: user nfsnobody: Initialization failed: Namespace '': mkdir(/home/nfsnobody/mailboxes) failed: Permission denied (euid=65534(nfsnobody) egid=65534(nfsnobody) missing +w perm: /home, dir owned by 0:0 mode=0755)
doveadm(nfsnobody): Error: User init failed
$ 

Version-Release number of selected component (if applicable):
dovecot-2.2.10-4.el7_0.1.x86_64

How reproducible:
Everytime, see above and below.

Actual results:
Default setting of first_valid_uid = 500 conflicts with system users.

Expected results:
Change default from 500 to 1000 to avoid conflicts with system users.