auto{conf,make} kernel-related macros consistency

 - add AC_DEFINEs for 2 kernels which were missing them.
 - declare AM_CONDITIONALs for all kernels we know about + move
   declaration at a more sensible location in the file.
zfs_arc: support for zfsonlinux

The plugin is structured for Solaris & FreeBSD's get_kstat_value() &
sysctlbyname() functions, which return a single metric based on its
name. ZFS-on-Linux publishes all the stats together in a file in /proc,
which is quite a different approach.
So the idea is to simulate the Solaris/FreeBSD way of doing by using
llist_search() on a linked-list containing all the key/values from the
/proc file. This prevents having to parse it over and over again for
each metric we request.
zfs_arc: support for zfsonlinux

fix alocation schemes so the plugin actually works
 - specify type llist_t kstat_t, so we can easily pass in our parsed data
 - read the file into one chunk of memory so we have the values available
 - count the number of lines to allocate the long pointer array
 - parse the buffer line by line.
zfs_arc: pass reference in llist instead of value

This allows the actual values from getting stored, not the pointer
addresses.
zfs_arc: style consistency
zfs_arc: copyright & attribution
rpm specfile: enable zfs_arc by default
zfs_arc: pass reference in llist instead of value

This avoids using a void pointer, which size depends on the
architecture.

Thanks to @dothebart for pointing this out!
zfs_arc: bugfix: increment array index

also avoid recycling the previous iterator, for the sake of better code
understanding.
New InternalStatistics
curl*: remove unneeded option declarations

They are equivalent to CURLOPT_USERPWD used a couple of line above and
aren't supported by curl versions older than 7.19.1
curl*: use CURLOPT_USERNAME if libcurl is recent enough

CURLOPT_USERNAME and CURLOPT_PASSWORD were introduced in curl 7.19.1.
They supersed CURLOPT_USERPWD which couldn't handle colons in the
username or password.
misc additions to curl* plugins doc & examples
add the prefix feature to the src
refactor prefix feature - cast directly to service_buffer
handle prefix=null
Fix indentation, change Prefix to EventServicePrefix
Add documentation about the plugin in the pod file
ignore EventServicePrefix when its empty ("")
check the status of cf_util_get_string