remove router.c internal functions from router.h

These functions (group A):

A
const char *router_get_description(char *buf, const routerinfo_t *ri);
const char *node_get_description(char *buf, const node_t *node);
const char *routerstatus_get_description(char *buf, const routerstatus_t *rs);
const char *extend_info_get_description(char *buf, const extend_info_t *ei);

Do the same as these (group B):

B
const char *router_describe(const routerinfo_t *ri);
const char *node_describe(const node_t *node);
const char *routerstatus_describe(const routerstatus_t *ri);
const char *extend_info_describe(const extend_info_t *ei);

With the difference that those last allocate a buffer, instead of forcing the caller to create and pass the buffer as a parameter.

The functions from group B are an abstraction to the ones from group A: they are better because they always generate buffers of enough size (the size is NODE_DESC_BUF_LEN). So, we should avoid using group A.

By now, both groups are declared in the header, and there is only one use of a function of group A (router_get_description is used on dirserv.c).

Also, all those functions are abstractions to format_node_description, that should also not be used externally, so we could also remove this one from the header.

The constant NODE_DESC_BUF_LEN is not necessary externally either.

Trac:
Username: valentecaio