api: versioned symbols in libgfapi.so for compatibility

Use versioned symbols to keep libgfapi at libgfapi.so.0.0.0

Some nits uncovered:

+ there are a couple functions declared that do not have an
  associated definition, e.g. glfs_truncate(), glfs_caller_specific_init()

+ there are seven private/internal functions used by heal/src/glfsheal
  and the gfapi master xlator (glfs-master.c): glfs_loc_touchup(),
  glfs_active_subvol(), and glfs_subvol_done(), glfs_init_done(),
  glfs_resolve_at(), glfs_free_from_ctx(), and glfs_new_from_ctx();
  which are not declared in glfs.h;

+ for this initial pass at versioned symbols, we use the earliest version
  of all public symbols, i.e. those for which there are declarations in
  glfs.h or glfs-handles.h.
  Further investigation as we do backports to 3.6, 3.4, and 3.4
  will be required to determine if older implementations need to
  be preserved (forward ported) and their associated alias(es) and
  symbol version(s) defined.

FWIW, we should consider linking all of our libraries with a map, it'll
result in a cleaner ABI. Perhaps something for an intern to do or a
Google Summer of Code project.

Change-Id: I499456807a5cd26acb39843216ece4276f8e9b84
BUG: 1160709
Signed-off-by: Kaleb S. KEITHLEY <kkeithle@redhat.com>
Reviewed-on: http://review.gluster.org/9036
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Tested-by: Niels de Vos <ndevos@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>