Log Message:
Add pnut: C compiler written in POSIX shell and generating POSIX shell scripts

Pnut compiles a reasonably large subset of C99 to human-readable POSIX
shell scripts. It can be used to generate portable shell scripts without
having to write shell.

Its main uses are:

- As a transpiler to write portable shell scripts in C
- As a way to bootstrap a compiler written in C with an executable
  version that is still human readable

Main features:

- No new language to learn -- C code in, shell code out
- The human-readable shell script is easy to read and understand.
- A runtime library including file I/O and dynamic memory allocations
- A preprocessor
- Integrates easily with existing shell scripts