Update spec file to enable systemd for RHEL7

Signed-off-by: Ben Allen <bsallen@alcf.anl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2355
Accept kernel source dir(s) specified by ./configure

This adds ability to set the location of the kernel via defines
when building from the spec files.  This is useful when building
against a kernel installed in a non-standard location.

Signed-off-by: Turbo Fredriksson <turbo@bayour.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1874
Do not export pool to prevent cache from been removed

Signed-off-by: Derek Dai <daiderek@gmail.com>
Signed-off-by: Turbo Fredriksson <turbo@bayour.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2353
Expand the description of scan-related and other parameters.

Document that the scan-related parameters are, in fact, applicable only
to scrub and/or resilver operations as appropriate.

Expand a few of the prefetch-related descriptions.

Add clarification to other module parameters.