Review GNATCOLL.Config testing and implementation

* Ignore all leading and trailing whitespaces (not only space and CR)
* Ensure Ini_Parser state is reset on call to Open
* Ensure comment and section declarations can have leading and/or
  trailing whitespaces.
* Ignore non valid patterns for GNATCOLL.Templates when attempting $HOME
  substitution.
* Ensure that '#' in section name is handled correctly
* Add test

Part of R307-006

Change-Id: I2fe5742baf069140ac2c4f58f89eeff0d23f22c4