Environment overhaul.

Environment overhaul.

Store XXXdirs files in ~/.PATH and add subdirectories for SYSTEM and
ARCHITECTURE under there.
Remove duplicate entries from paths set in PATH.bashrc.
Also read XXXdirs.pre and XXXdirs.post.
.pre entries are guaranteed to come before whatever was already defined
or is defined with XXXdirs.
.post entries are guaranteed to be present but may not necessarily be
last if they were already defined.

Add paths under opt to default XXXdirs.pre files.
Add /lib to libdirs.post on Solaris as the Solaris linker uses the value
of LD_LIBRARY_PATH to override system defaults.