more complete support for implicit thread/interpreter pointer,

more complete support for implicit thread/interpreter pointer,
enabled via -DPERL_IMPLICIT_CONTEXT (all changes are noops
without that enabled):
  - USE_THREADS now enables PERL_IMPLICIT_CONTEXT, so dTHR
    is a noop; tests pass on Solaris; should be faster now!
  - MULTIPLICITY has been tested with and without
    PERL_IMPLICIT_CONTEXT on Solaris
  - improved function database now merged with embed.pl
  - everything except the varargs functions have foo(a,b,c) macros
    to provide compatibility
  - varargs functions default to compatibility variants that
    get the context pointer using dTHX
  - there should be almost no source compatibility issues as a
    result of all this
  - dl_foo.xs changes other than dl_dlopen.xs untested
  - still needs documentation, fixups for win32 etc
Next step: migrate most non-mutex variables from perlvars.h
to intrpvar.h

p4raw-id: //depot/perl@3524