#759 Add operating system to rexxc' image metadata part

[Rony G. Flatscher]

It would be helpful for maintenance and deployment utilities, if the metadata in a rexxc'd image file would also contain a hint to the operating system that hosted the creating of the rexxc'd image.

One possibility would be to use the first three letters of the operating system and store it in the new four byte reserved field, e.g. for Windows "Win\0", for Linux "Lin\0" and the like.