Setting the const attribute to strings of API

Because clients are often using "const char *" instead
of just "char *" in their internal API, it make sense
to offer this new API. Their are two main reasons:
- passing a "char*" for a "const char*" is okay and
doesn't produce warning.
- the developper doesn't have to ask himself if removing
the const with a cast is okay or not.

Change-Id: I4640def0d512bad63049b42a0cf449ff6c9f7b08