gdbsupport: fix memory leak in create_file_handler when re-using file handler

gdbsupport: fix memory leak in create_file_handler when re-using file handler

ASan made me notice a memory leak, where the memory tied to the file
handle name string wasn't freed.  When register a file handler with an
fd that is already registered, we re-use the file_handler object, so we
ended up creating a new std::string object and overwriting the
file_handler::name pointer, without free-ing the old std::string.

Fix this by allocating file_handler with new, deleting it with
delete, and making file_handler::name not a pointer.

Change-Id: Ie304cc78ab5ae5dfad9a1366e9890c09de651f43