Set default _initconfdir directory

The _initconfdir macro is normally provided by global rpm macros
file for use in the spec file. However, older distributions such
as CentOS 6 do not define it. To prevent a build failure in this
case the spec file has been updated to use a reasonable default
when the value is undefined.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3617