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>

include/map.h

@@ -36,15 +36,15 @@
 #include "types.h"
 
 /*
- * Width of the body of a map using PERSEC characters per sector.
+ * Width of the body of a map using @persec characters per sector.
  *
  * One row shows WORLD_X/2 sectors, separated by one space.  Requires
- * WORLD_X/2 * (PERSEC+1) - 1 characters.
+ * WORLD_X/2 * (@persec+1) - 1 characters.
  *
  * Every other row is indented so that the center of the first sector
  * is aligned with the space separating the first two sectors in the
- * adjacent rows.  For odd PERSEC, that's (PERSEC+1)/2 additional
- * characters.  For even PERSEC, it's either PERSEC/2 or PERSEC/2 + 1,
+ * adjacent rows.  For odd @persec, that's (@persec+1)/2 additional
+ * characters.  For even @persec, it's either @persec/2 or @persec/2 + 1,
  * depending on whether we align the character left or right of the
  * center with the space (the map will look rather odd either way).
  *