[SSSD] SELinux login management

[Jakub Hrozek]

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Adds a new option -Z to sss_useradd and sss_usermod. This option allows
user to specify the SELinux login context for the user. On deleting the
user with sss_userdel, the login mapping is deleted, so subsequent
adding of the same user would result in the default login context unless
- -Z is specified again.

MLS security is not supported as of this patch.

Also adds explicit build dependency on libselinux-devel - it is dragged
in by krb5-devel currently, but I think the dependency should be listed
since we directly use functions from libselinux to set homedir contexts.

Addresses: #230
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