Util: Improve code to get connection credentials
    
    Adds support to get SELINUX context and make code more abstract so
    that struct ucred (if availale) can be used w/o redefining uid,gid,pid to
    int32. Also gives a layer of indirection that may come handy if we want
    to improve the code further in the future.
    
    Signed-off-by: Lukas Slebodnik <lslebodn@redhat.com>
    Reviewed-by: Michal Židek <mzidek@redhat.com>