Give warning when MODULEPATH contains a trailing slash

[fgeorgatos]

We should inspect the $MODULEPATH and report if it deviates from "normal", because trailing slashes trigger different kind of bugs:

• On environment-modules-C we see that module list shows `/GCC* etc
• On environment-modules-tcl we see the opposite: the first character is truncated

I think the experience calls for a regex test and spitting out warnings; see related #689
(this is important to avoid the situation that people misreport bugs on EB side)

[boegel]

I've never been able to reproduce any of the issues outside of EasyBuild, whether I use a trailing slash in $MODULEPATH or not.
In #739, we make sure that module names don't start with a slash, which seems to be triggered by $MODULEPATH ending in / (but only under certain conditions!), but it would still be worthwhile to check this to avoid stuff going haywire because of a single character...