add module-search-path-headers configuration option to control how modules set search paths to header files

[lexming]

Depends on:

• refactor generation of required environment variables in module files #4653

This PR adds a new build option and easyconfig parameter called module-search-path-headers to control what environment variables will be added to module files with search paths to headers:

• none: (new setting) no variables added to module files with paths to headers
• CPATH: (default and current setting) modules set CPATH on load
• INCLUDE_PATHS: (new setting) modules set variables C_INCLUDE_PATH, CPLUS_INCLUDE_PATH, OBJC_INCLUDE_PATH on load with paths to headers

New test added to easyblock suite for this new option.