Add perror to libcmini

[pietvo]

I needed perror in my project. There was none in libcmini, so I decided to write it myself. I submit it hereby for inclusion in the library.

[th-otto]

Thank you. But i have a few comment:

• <sys/errno.h> does not exist. That should be just <errno.h>
• <unistd.h> is not needed there at all

[pietvo]

Thanks for your comment. In fact, <errno.h> isn't needed either, because what it supplies is accessed by strerror.

I have done some cleanup and will push a new version.

[th-otto]

errno.h is needed for the declaration of the errno variable.

Edit: i just see that it is also declared in stdio.h. But imho this is wrong.

[pietvo]

OK, I'll put it back. It is a bit confusing. On my Mac, the man page says that it is in <sys.errno.h> and I see in some compiler setups that <errno.h> includes <sys.errno.h> or the other way around.