Create davmail user/group per new conventions in RPM 4.19

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When trying to install the DavMail RPM from the Open Build Service repository on openSUSE, Zypper displays the following error message:

Problem: 1: nothing provides 'group(davmail)' needed by the to be installed davmail-6.2.2-3.4.noarch
Solution 1: do not install davmail-6.2.2-3.4.noarch
Solution 2: break davmail-6.2.2-3.4.noarch by ignoring some of its dependencies

Choose from above solutions by number or cancel [1/2/c/d/?] (c):

A similar problem probably occurs when trying to install the DavMail packages on any OS that uses RPM >= 4.19, and is due to a change in how that version of RPM handles users and groups. This issue and the possible workarounds for packagers is discussed in this openSUSE Factory mailing list post.

I will submit a patch shortly via OBS.

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Patch submitted as Request 1176523.