Move env setup code from init script to autoinit process

[xdelaruelle]

Move as much environment setup code as possible from the shell initialization scripts to the autoinit internal process of modulecmd.tcl.

It targets:

• module function exports
• PATH setup for Modules bin location
• MANPATH setup for Modules manpages
• Source of completion scripts

Within modulecmd.tcl it will be simpler to perform these initialization tasks:

• same code for every kind of shell
• simpler in Tcl to check for duplicate entries
• code already available to escape special characters ($, whitespace, etc)