These functions create MIME fields in a MIME header. The header is specified by the combination of
the buffer bufp and a location hdr. The header must be either created such as by
TSMimeHdrCreate() or be an existing header found via a function such as
TSMimeHdrFieldCreate() creates a completely empty field which must be named before being used
in a header, usually via
TSMimeHdrFieldNameSet(). It is almost always more convenient to use
TSMimeHdrFieldCreateNamed() which combines these two steps, creating the field and then
setting the name to name.
For both functions a reference to the new field is returned via arg:out.
The field created is not in a header even though it is in the same buffer. It can be added to a
TSMimeHdrFieldAppend(). The field also has no value, only a name. If a value is
needed it must be added explicitly with a function such as