Import modglue-1.19 as wip/modglue.

The libModglue library is a C++ library with classes for forking external
processes and asynchronous reading from streams. It takes away the burden
of all subtleties involving the Unix fork call. The asynchronous read
facility enables one to read on multiple input streams at the same time,
without losing any of the standard C++ stream facilities.
There are also several small additional programs bundled with modglue,
such as a program to add readline capability to any command line program.
Moreover, the modglue library extends the idea of standard Unix pipes by
providing a general framework for the creation of new processes with more
than just the standard stdin/stdout/stderr pipes.