guestfish: Enable grouping in string lists

guestfish: Enable grouping in string lists

This change adds the ability to group entries in a string list with single
quotes. So the string:
  "'foo bar'"
becomes 1 token rather than 2. Consequently single quotes must now be escaped:
  "\'"
resolves to a literal single quote.

Incidentally, this change also alters another, probably unintentional behaviour
of the previous implementation, in that tokens are separated by any amount of
whitespace rather than a single whitespace character. I.e.:
  "a  b"
resolves to:
  'a' 'b'
rather than:
  'a' '' 'b'
That last syntax can be used if an empty argument is still desired. Whitespace
is now also defined to include tabs.

parse_string_list can also now fail if it contains an unmatched open quote.