Refactor handle_qxfer_libraries and friends to use std::string

Refactor handle_qxfer_libraries and friends to use std::string

Using std::string in handle_qxfer_libraries and friends allow to
simplify the code.  We don't have to manually free the buffer, and we
don't have to pre-compute the required space.

gdb/gdbserver/ChangeLog:

	* server.c (accumulate_file_name_length): Remove.
	(emit_dll_description): Adjust to std::string change.
	(handle_qxfer_libraries): Use std::string to hold document.