Promote more Isabelle concurrency code

Portable gets slightly richer with

make_counter : {inc:int,init:int} -> unit -> int
syncref : 'a -> {get:unit->'a, upd : ('a -> 'b * 'a) -> 'b}

Under Moscow ML these map to obvious operations on references. Under
Poly/ML these map to synchronised variables that are operated on
atomically.

Empty command-line Holmake only builds first target in Holmakefile.

If there is no Holmakefile, or if there is a Holmakefile with no
targets, then the old behaviour of attempting to build everything
possible happens.

Documented in release notes and DESCRIPTION.

Many of the distribution's Holmakefiles were harmed in the creation of
this commit, but, for the reasons given in the release notes, I don't
think this should happen that often for typical user developments.

Closes #175

Better document our use of the Susp code from Moscow ML.

Unfortunately, a Moscow ML 2.01 bug (now reported) means we end up
having to use a new name (HOLSusp) to get the dependencies working
properly. Without this change, building the standard kernel on Moscow
ML would always recompile Thm.sml, seemingly because it couldn't see
the non-existent Susp.ui in SIGOBJ (this file only appears there in the
Poly/ML build). There may still be a Holmake bug here too.

New "portable" subdirs per ML implementations in src/portableML.

Hope to do away with Poly/ML specific code in tools-poly eventually,
moving it into src/portableML/poly. tools/build-sequence now records
that the implementation-specific directories have to be included.