Make xml_escape_text return an std::string

Make xml_escape_text return an std::string

This is a simple replacement, it allows removing some manual free'ing in
the callers.

gdb/ChangeLog:

	* common/buffer.c (buffer_xml_printf): Adjust.
	* common/xml-utils.c (xml_escape_text): Change return type to
	std::string, update code accordingly.
	* common/xml-utils.h (xml_escape_text): Change return type to
	std::string.
	* rs6000-aix-tdep.c (rs6000_aix_shared_library_to_xml): Adjust.
	* windows-tdep.c (windows_xfer_shared_library): Adjust.
	* unittests/xml-utils-selftests.c (test_xml_escape_text):
	Adjust.

gdb/gdbserver/ChangeLog:

	* linux-low.c (linux_qxfer_libraries_svr4): Adjust to change of
	return type of xml_escape_text.
	* server.c (emit_dll_description): Likewise.