banned.h: mark `strtok()` and `strtok_r()` as banned

`strtok()` has a couple of drawbacks that make it undesirable to have
any new instances. In addition to being thread-unsafe, it also
encourages confusing data flows, where `strtok()` may be called from
multiple functions with its first argument as NULL, making it unclear
from the immediate context which string is being tokenized.

Now that we have removed all instances of `strtok()` from the tree,
let's ban `strtok()` to avoid introducing new ones in the future. If new
callers should arise, they are encouraged to use
`string_list_split_in_place()` (and `string_list_remove_empty_items()`,
if applicable).

string_list_split_in_place() is not a perfect drop-in replacement
for `strtok_r()`, particularly if the caller is processing a string with
an arbitrary number of tokens, and wants to process each token one at a
time.

But there are no instances of this in Git's tree which are more
well-suited to `strtok_r()` than the friendlier
`string_list_split_in_place()`, so ban `strtok_r()`, too.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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