Clean up our cleaning

'make clean' and 'make maintainer-clean' still leave around a fair
number of droppings, prior to this commit.

We were not descending into the 'tests' top-level directory while
cleaning.  Furthermore, tests/opr/Makefile needed $(LT_CLEAN), and
tests/rx/Makefile needed to spell it correctly.

The libtoolization places a lot of files to be removed in the
'pristine' target.

The processing used to implement the =include directive in the pod
sources for the man pages leaves around the non-.in versions of
files; we should clean that up in the 'pristine' target as well.

The 'pristine' target should likewise remove the man pages which
are generated from the pod files.

Additionally, the documentation build uses a Doxyfile which is
output by configure; that should be removed (if present) by the
'distclean' target.

When hcrypto was converted to libtool, the use of ${OBJECTS} in
the clean target was missed, so we were leaving around most of the
actual object files -- $(LT_CLEAN) does not handle this for us.
Change the rule to remove *.o as is done elsewhere.

The conversion of libafsrpc to libtool added a convenience library
libafsrpc_sys.la, and changed how syscall.o was generated on
most architectures, to be the result of compiling an empty .c file
(instead of just an empty .o file).  This introduced a new
intermediate file, syscall.c, which must be cleaned up.

tvolser was only listing volserver and not vos in its list of
executables to remove while cleaning.

The conversion of venus/test to libtool was not done quite right.
Makefile.libtool and the .lo suffix are only needed when libtool
is being used to link *libraries*; just Makefile.pthread suffices
when libtool is being used to link executables.  As such, remove
the inclusion of Makefile.libtool, and change the .lo targets back
to regular .o ones, and add back *.o to the list of files to remove
in the 'clean' target (it was needed there even without the
other changes to that Makefile).

Change-Id: Ifbc3eee4ad2dce54df991301bc5edd11eb29a24a