Rune - Stabilization, flesh out core classes

Rune - Stabilization, flesh out core classes

* Add many more constants.

* Rename class Fd to class Fs and rearrange the methods.  Add additional
  classes.  Start working on Stream and StreamPair.  Somewhat hide the
  low-level pipe() and socketpair() calls.  Give StreamPair similar methods
  that make life easier.

* Fix a few cases where SGF_ADDRUSED and EXF_ADDRUSED were not being
  propagated properly by the resolver.  Hack the context code to check
  the arguments for ADDRUSED to trigger SRSGET/SRSPUT in the procedure's
  main context.  Fix a related bug.

* Properly propagate ADDRUSED and ADDROF in exp.exp (TOK_DOT) expressions.
  This introduces a bit of a performance regression due to extra LVRSGETs
  which will be fixed later.  Fixes a number of lvalue-related issues.

* Fix a ref-count issue revealed by a more sophisticated tests/fd_pipe.d
  Also test fgetbuf(), a high-performance descriptor data streamer.