Feature: the execute functions now support custom pipes.

Feature: the execute functions now support custom pipes.

This is necessary for the parent process to directly pipe date to/from a child process.
One of the intended goals is for something like the controller program to directly execute a bash script from a string generated by the parent (where this is no file).
The standard input can be used by the child process as pipe data.

When a pipe is passed then the execute functions are asynchronous instead of blocking.
The caller must properly handling all blocking operations as appropriate.

The pipe functionality is optional.