sockets: must include header files in tarball

The sockets provider .h files must (eventually) be listed in a
_SOURCES or EXTRA_DIST Makefile.am value so that Automake will
include them in distribution tarballs.

This commit removes the sym link for indexer.h, because there was no
-I set to prov/src/sockets to pick up that file, so the build was just
picking up include/indexer.h, anyway.

This raises another point: should indexer.c be sym linked to
prov/src/sockets?  It seems like this is another facett of the ongoing
"common sources" debate on #207.

This is part of the fix for #219.

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>
Makefile.am: don't distribute the DIRECT header files

Separate installing DIRECT header files from including them in the
tarball.

* By adding "nodist_" to the front of the existing
  AM_CONDITIONAL-protected rdmainclude_HEADERS AM list of DIRECT
  headers, Automake will install the headers when directed.  However,
  Automake will *not* automatically include these files in "make
  dist"-generated tarballs.
* Each provider is responsible for adding their DIRECT header files to
  EXTRA_DIST.

Each provider is responsible for adding its DIRECT header files to
EXTRA_DIST.  We can't list the parameterized list of DIRECT header
files to be installed to the dist list because the value of
$(PROVIDER_DIRECT) is irrelevant (and may well be empty).

Fixes #219

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>