Fix quoting of font-family in 'hfy-family'

Fix quoting of font-family in 'hfy-family'

Running "M-x htmlfontify-buffer" in one buffer, the exported HTML contains
lines like:

  body, pre { text-decoration: none;  font-family: Migu 2M;  font-stretch:
  normal;  font-weight: 500;  font-style: normal;  color: #ffffff;
  background: #000000;  font-size: 15pt; }

Standards-compliant web browsers should ignore this font-family.

MDN Web Docs says:
https://developer.mozilla.org/en-US/docs/Web/CSS/font-family

  Valid family names
  Font family names must either be given quoted as strings, or unquoted as
  a sequence of one or more identifiers. This means that punctuation
  characters and digits at the start of each token must be escaped in
  unquoted font family names.

  It is a good practice to quote font family names that contain white
  space, digits, or punctuation characters other than hyphens.

An unquoted font-family is valid as long as it doesn't start with a digit,
but MDN Web Docs also says:

  The following example is technically valid but is not recommended:
  font-family: Gill Sans Extrabold, sans-serif;

So it makes sense to quote all font-family.

* lisp/htmlfontify.el (hfy-family): Quote 'font-family'.  (Bug#62054)