Improved process execution

Groovy has numerous features for improving Java's external process execution. This issue is about packaging those features up in a more concise way. This supercedes GROOVY-5049 (thanks goes to Uri Moszkowicz for raising many of the issues).

So for this Perl example here:

if (`ls *.ext1` =~ /\S+/) {
  ...
} elsif (`ls *.ext2` =~ /\S+/) {
  ...
}

Currently a solution might look like this:

def sb1 = new StringBuffer()
def process1 = "ls *.ext1".execute()
process1.waitForProcessOutput(sb1, sb1)

def sb2 = new StringBuffer()
def process2 = "ls *.ext2".execute()
process2.waitForProcessOutput(sb2, sb2)

if (sb1 =~ /\S+/) {
  ...
else if (sb2 =~ /\S+/) {
  ...
}

The intention would be to provide something like:

if (Process.execute('ls *.ext1') =~ /\S+/) {
  ...
else if (Process.execute('ls *.ext2') =~ /\S+/) {
  ...
}

as a shorter alternative (perhaps even shorter after a static import).

For more complicated scenarios we might have:

Process.execute(command: 'ls *.ext1', timeout: 5000,
    redirectErrorStream: true, wasKilled: { println 'FAILED!' })

This is combining ideas from GROOVY-4991 and GROOVY-4992.

The intention would be to use Java's ProcessBuilder if needed and see if we can learn anything from org.jruby.util.ShellLauncher which does things like adds 'cmd /c' in front of internal commands on Windows and is are of standard pasth prefixes, e.g. /usr/bin and handles channels for the inputs and outputs if needed.