Implement dash as bootstrap shell

[Jonathan Perkin <jperkin%joyent.com@localhost>]

I'd like to make shells/dash a bootstrap shell, with the aim of
eventually replacing shells/pdksh.  This commit adds initial support,
adding a --with-dash bootstrap argument to enable it.

  https://github.com/joyent/pkgsrc/commit/6173446

The primary motivation is twofold:

 - shells/pdksh is horribly broken, with intermittent failures on at
   least SunOS.

 - shells/dash is significantly faster than bash, the default shell on
   a number of platforms.

I've been running variations of this diff in bulk builds for a while
now, and dash is only a few packages behind the bash-based builds due
to some non-POSIX constructs.  It has been tested on illumos, Solaris
9, and OSX with and without --with-dash.  Some older bulk build
results that I performed with various shells are here:

  https://gist.github.com/jperkin/992c375d067c8960541d

The only change that could possibly affect users will be the addition
of:

  CONFIGURE_ARGS+= --with-defshell=${TOOLS_PLATFORM.sh}

to the default set in devel/bmake, to accommodate the selection of a
bootstrap shell.  In theory this shouldn't cause any problems, and any
issues that do occur due to it are likely to be real bugs that need
fixing anyway.

I'd like to get this in before the freeze to give users the chance to
test it, and I'd certainly like to use it for our 2015Q4 package sets.

Opinions?

-- 
Jonathan Perkin  -  Joyent, Inc.  -  www.joyent.com