mkdir when vis output gets placed in a subdir

This way we don't have to remember to pre-create subdirectories before
launching jobs with more organized output locations.
Include <errno.h> to be safe

My glibc pulls it in from those other headers, but I can't find
anything in the spec guaranteeing that.
Merge pull request #135 from grinsfem/vis_output_mkdir

mkdir when vis output gets placed in a subdir