header-only Windows implementation of the <unistd.h> header.
tested on the following compilers:
- Visual Studio
 - Clang for Windows (clang-cl)
 - GCC (MinGW)
 
implements the following functions:
- access
 - chdir
 - close
 - dup
 - dup2
 - execl
 - execle
 - execlp
 - execp
 - execpe
 - execpp
 - rmdir
 - unlink