Fix various generic kmod RPM spec issues.

There are a number of issues with the generic kmod RPM spec in its
current state:

• The "%{id_u}" macro seems to not be available on some systems (e.g. Debian squeeze). It appears it has been deprecated. Use "${id} -u" instead.
• The way the "--with-linux=" configure option is generated in the non-RHEL/Fedora case is completely wrong with various newline and escaping issues (also, $kernel_version is not available in the generator context).

The second issue made the generator shell snippet (almost) silently
fail, which under specific circumstances can result in broken builds
against the wrong kernel sources.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1416