Initialize path-like env variable to a configured value when first time set #572
Description
When path-like environment variable is unset then some path entry is added for the first time, it is desired in some case to be able to initialize the variable to a configured value then add to it the first path entry.
This is for instance the case with MANPATH variable, where man command relies on a configuration file when MANPATH is unset. Then only relies on the content of MANPATH when set. For this environment variable it is important to defined its initial value to the list of paths used when this variable is unset, before adding it the path entry required by a modulefile.
Such feature was part of Modules 3 but dedicated to the management of MANPATH environment variable.
The idea here is to provide a mechanism that could be used for any path-like environment variable and triggered when env var is unset and a call to append-path or prepend-path is made for it.