Change comment style to use @foo rather than FOO

Change comment style to use @foo rather than FOO

... when referring to a function's parameter or a struct/union's
member.

The idea of using FOO comes from the GNU coding standards:

    The comment on a function is much clearer if you use the argument
    names to speak about the argument values.  The variable name
    itself should be lower case, but write it in upper case when you
    are speaking about the value rather than the variable itself.
    Thus, "the inode number NODE_NUM" rather than "an inode".

Upcasing names is problematic for a case-sensitive language like C,
because it can create ambiguity.  Moreover, it's too much shouting for
my taste.

GTK-Doc's convention to prefix the identifier with @ makes references
to variables stand out nicely.  The rest of the GTK-Doc conventions
make no sense for us, however.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>