DTrace-independent CTF.

DTrace-independent CTF.

These largely cosmetic changes remove mention of DTrace from the CTF code,
making it clear that it is freely usable by non-DTrace consumers.

The changes are:

 - dtrace_ctf.ko is now named ctf.ko, and is stored in kernel/ctf rather than
   kernel/dtrace, controlled by a new CONFIG_CTF Kconfig option select'ed by
   CONFIG_DTRACE.  (CONFIG_DT_DISABLE_CTF, being largely a DTrace debugging
   option, remains under DTrace configure control).  The function used to
   trigger loading of ctf.ko has changed name similarly, from
   dtrace_ctf_forceload() to ctf_forceload().

 - The CTF section names have changed, from .dtrace_ctf.* to .ctf.* (which as a
   bonus is more obviously related to the .ctf directory long used to store the
   CTF data during the build process).

 - The shared CTF repository is now stored in .ctf.shared_ctf instead of
   .dtrace_ctf.dtrace_ctf, making its intended use somewhat clearer.

These changes depend on a suitably changed libdtrace-ctf: a suitably changed
userspace is needed to take advantage of them.  The dtrace-kernel-interface is
bumped accordingly.

Signed-off-by: Nick Alcock <nick.alcock@oracle.com>