AIX systems need to use the SYSV stty(1) variant.

In generating CSHENV prompts, I used stty(1) to find the width
of the current terminal, at sourcing time.  SYSV and BSD use
different output formats.  Presently I have tcshrc.prompt.defs
default to 80 columns when I don't know how to parse stty(1)'s
output.  AIX is known to be a SYSV variant, and I've added a
case to handle this.
Merge branch 'aix-tweaks' into initial-rework