Implement BROWSE on Mac OS X.

On OS X, `OS_Browse` only attempts to use `/usr/bin/open`; on other
POSIX platforms, "xdg-open" and "x-www-browser" are used as before.
Clean up browser calls for POSIX platforms.

The `system(3)` call in `Try_Browser` has been replaced with `fork(2)`
and `execlp(3)`. As such, the call string needn't be constructed and
failed calls no longer print their output to the console.
REWORD API Revamp (See http://issue.cc/r3/1990)

Requested changes (requester):
* Binary support, and working tag support (BrianH)
* /case for case-sensitive comparison (BrianH)
* Specifying leading and trailing delimiters with /escape (i.e. for
HTML/XML entities) (BrianH)
* Being able to specify none as an /escape, equivalent to "" now
(HostileFork)
* Evaluating value expressions directly in a block spec, rather than
requiring the spec to be reduced, and /only to turn that off
(rgchris, BrianH, prompted by GrahamChiu's RebolBot code)
* Evaluating block values as code like functions, to cut down on
function
creation overhead, and /only to turn that off (rgchris)
* Special-case treatment of set-word keys as if they were regular words,
so block and object specs can be compatible (BrianH)
* Special-case treatment of lit-word keys as if they were regular words,
to be compatible with fully reduced specs (BrianH)
* Directly using values specs that already match what we need
internally,
to save overhead (BrianH)
* Better-phrased doc strings that are short enough to not wrap on the
Windows console (BrianH)
Merge pull request rebol/r3#101 (0branch/browse-macosx) into community

* 0branch/browse-macosx:
  Clean up browser calls for POSIX platforms.
  Implement BROWSE on Mac OS X.
Merge pull request rebol/r3#103 (BrianHawley/wish-1990-reword-rework) into community

* BrianHawley/wish-1990-reword-rework:
  REWORD API Revamp (See http://issue.cc/r3/1990)