Doxygen: Make output more C-like

There are a couple of options we should set to make our doxygen output more C-like and less C++-like.

The TYPEDEF_HIDES_STRUCT option tells doxygen to handle typedef struct foo_t foo_t by making foo_t and struct foo_t synonymous. This lets doxygen find documentation that it would otherwise miss: otherwise, if it sees documentation for "int func(foo_t *)" and a prototype for "int func(struct foo_t)", it will think that the prototype is undocumented.

The HIDE_SCOPE_NAMES option tells doxygen to describe a member "member" of a struct "container" as "member", not "container::member".