Motivation: we want to load external configuration file based on the
state of some environment's variable (for example) and this file may not
exist.
For the nature of source keyword, the sourcing of a configuration file
is performed, neglecting the fact that the entry is inside a
contictional structure and that the condition is not verified (and the
configuration file doesn't exist). With this context the conf tool
exits with error.

Solution: introduce a newer keyword, similar to source one but where the
conf tool doesn't return an error, but instead a warning, if the
configuration file doesn't exist. This newer keyword is osource

Syntax:

	"osource" <prompt>