Makefile.SH: rationalise object list variables

Makefile.SH: rationalise object list variables

There are a number of variables in the generated Makefile, such as
'obj' and 'minindt_obj', that enumerate various subsets of the object
files that need to be compiled and linked.

Rename and reorganise these vars slightly, to make the next commit
simpler. In particular it now splits the object files in into 3 sets:
common (av.o etc), those used just by miniperl (opmini.o etc)
and those used just by perl (op.o etc).

Should be no functional changes.

The changes to os2/Makefile.SHs have been done blind. Does anyone still
use OS2?