The exec logic calls through these methods to build a command line
that runs the windows cmd.exe to launch the desired executable.
Unfortunately this logic has two problems:

* The eventual jnr-posix `exec` implementation uses CreateProcess,
  which requires a full path to cmd.exe. Because all other paths
  use the JDK ProcessBuilder, we did not perform a search for this
  executable and the CreateProcess fails.
* The desired executable does run through the path and extension
  search logic, but the resulting discovered executable (such as
  .cmd or .bat) is not used in the cmd.exe command line.

This patch makes the following changes:

* Perform the executable search for cmd.exe, so we can provide a
  full path to CreateProcess
* Use the result of the executable search in place of the original
  desired executable name, so it will pick up non-exe files like
  .bat and .cmd.

See jruby#6745

On Windows, the default logic (from rubygems/defaults/jruby) will
see that we are running the build from within the 9.1.17.0 jar
file, and produce .bat wrappers that attempt to run JRuby using
a `java` command line. This breaks those .bat files, as described
in jruby#6745, because the `java` command line gets completely wrapped
in quotes and no longer can be run as a single executable.

This patch un-patches the default Gem.ruby so that it goes back to
just using `jruby` as the command line to launch JRuby.

This is a pretty nasty hack only necessary because the build runs
the gem installation from within the 9.1.17.0 jar file. If the
installs were run from a normal JRuby command line (i.e. the
current working copy's bin/jruby) the wrappers are generated
correctly.

global_bin seems to not be readable from within the block under
some circumstances.

We parse out single-string commands along this path, which may
result in a single script with embedded spaces being broken into
individual args. Here we check that the original rawArgs length
matches the parsed args length so we know we will only replace the
argv[0] executable and not the entire script.