(maint) Remove debug output from test.

A new test was added in a previous commit that used
cout to debug values being read in from a file.

The statement to output the lines should not have
been included in the test.
(maint) Allow split utility function to return empty entries.

There are valid cases for splitting a string and desiring empty
values returned in the vector.

For example, splitting "FOO=" (delimiter is '=') for an environment
variable would have previously return { "FOO" }.  This is still the
default behavior.

Now with an optional parameter, split can return { "FOO", "" }.
(CFACT-35) Implement environment merging for child processes.

Implement the merging of environment variables for the execution::execute
and execution::each_line functions.

This also sets LANG and LC_ALL to "C" if not explicitly specified to the
functions.