gdb: make inferior::m_args an std::string

gdb: make inferior::m_args an std::string

With the current code, both a NULL pointer and an empty string can mean
"no arguments".  We don't need this distinction.  Changing to a string
has the advantage that there is now a single state for that (an empty
string), which makes the code a bit simpler in my opinion.

Change-Id: Icdc622820f7869478791dbaa84b4a1c7fec21ced