Handle save/load of set-typed config items.

I'm about to introduce a configuration option which is really a _set_
of string values (rather than an ordered list), and I'm going to
represent it in Conf as a string->string map (since that's a data type
we already support) in which every key which exists at all maps to the
empty string.

This change modifies settings.c so that it can write out such options
into the saved session as a comma-separated list of the key strings
only, rather than the form 'string1=,string2=,string3=' which you'd
get if you just used the existing wmap().

(Reading the result back in turns out not to need a code change - the
existing code already does what we want if it's reading a list of
key=value pairs and one of them doesn't have an = sign at all.)

[originally from svn r10217]