Remove usage of the __id_u macro for portability.

This macro was removed from the default RPM macro file.  Interestly,
some of the arch specific macro's add it back it based on your distro
but it should not be counted on.  However, __id still exists and its
command line args have historically been fairly stable so we will
directly use %{__id} -un to get the user name.

zfs-modules.spec.in

@@ -206,7 +206,7 @@ Version:         %{version}
 Release:         %{release}
 License:         @LICENSE@
 URL:             git://eris.llnl.gov/zfs.git
-BuildRoot:       %{_tmppath}/%{name}-%{version}-%{release}-%(%{__id_u} -n)
+BuildRoot:       %{_tmppath}/%{name}-%{version}-%{release}-%(%{__id} -un)
 Source:          @PACKAGE@-%{version}.tar.gz
 Requires:        %{krequires}
 Requires:        %{splrequires}

zfs.spec.in

@@ -10,7 +10,7 @@ Version:         %{version}
 Release:         %{release}
 License:         CDDL
 URL:             git://eris.llnl.gov/zfs.git
-BuildRoot:       %{_tmppath}/%{name}-%{version}-%{release}-%(%{__id_u} -n)
+BuildRoot:       %{_tmppath}/%{name}-%{version}-%{release}-%(%{__id} -un)
 Source:          %{name}-%{version}.tar.gz
 Requires:        zlib
 BuildRequires:   zlib-devel