unify XML string escaping implementations

The current code base (608aab7b) has four implementations (that I have found so far) of XML string escaping:

• xml_string and friends in cmd/tools/gv2gxl.c
• xml_string and friends in cmd/smyrna/smyrna_utils.c
• xml_string and friends in lib/common/labels.c
• html_string and friends in plugin/core/gvrender_core_vml.c

The identical comments surrounding these implementations make it clear they were once copy-pasted from the same source. The lib/common one looks like the original and a comment in Smyrna even explains why this situation exists:

/* many of these functions are available in libcommon.
 * We cannot use those because dependencies cause a great
 * deal of libcommon to be brought in, which complicates
 * and makes possible conflicts as some parts of libcommon
 * rely on not using libcgraph.
 */

As far as I'm aware, this situation no longer exists. libcgraph is the only graph library supported and there is no sensible way to use libcommon without libcgraph. However, a naive attempt to undo the Smyrna code duplication fails because:

1. A number of libcommon headers rely on something else being #included prior to them. lib/common/utils.h that contains the libcommon prototype for xml_string cannot be #included without other libcommon headers being #included prior to it.
2. A number of libcommon typedefs/prototypes conflict with other ones in Smyrna headers.

The situation is pretty messy and may require diagnosing and undoing the conflicting definitions one-by-one. But I think it's worth doing in service of paring back the number of XML string escaping variants we're maintaining to 1.