Currently, contrib/oid2name doesn't bother to free() the memory that it

Currently, contrib/oid2name doesn't bother to free() the memory that it
malloc()'s. This isn't too serious (because oid2name is a short-lived
utility, so the memory will soon be returned to the OS on process
termination), but I still think it's poor style.

This patch changes oid2name so that it allocates memory on the stack
where possible and free()s the remaining heap-allocated memory. The
patch also fixes a typo a comment and adds 'const' qualifiers to a few
'char *' function parameters.

Neil Conway