This the first step in moving the collection of layer 2 netstats from
the low level driver to a central location, ie. gnrc_netif, to avoid
code duplication.

Use the `gnrc_netif_t::pid` member instead of the pid of the current thread when generating the the `gnrc_netif_hdr` in `gnrc_netif_ieee802154::_recv` function.

Useful in sniffer situations to see the signal strength of a received
packet. Also avoids the crash described in
https://github.com/RIOT-OS/applications/pull/54

This replaces the netif->state accesses in netif::recv with a netdev call
and the new netif RAWMODE flag

Set frame pending bit in IEEE 802.15.4 FCF when
GNRC_NETIF_HDR_FLAGS_MORE_DATA is set on the netif header on outbound
frames.

This adds a netdev recv call to indicate that the received frame should be
dropped when there is no buffer space available to store the frame.

With some minor hand-edits I used the following chain of commands:

```sh
git rm sys/include/net/gnrc/netdev.h
git grep --name-only -i netdev2 | \
        xargs sed -i -e 's/^\(NETDEV\)2\(.*\)\( [("]\)/\1\2 \3/g' \
                     -e 's/\(netdev\)2\(.*\)\( \/\*\*<\)/\1\2 \3/I' \
                     -e 's/\(netdev\)2/\1/gI'
git add -p
git commit --amend
git ls-tree --full-tree -r HEAD --name-only | \
        grep "netdev2" | xargs -I'{}' dirname '{}' | uniq | \
        grep "netdev2" | while read dir; do
                new_dir="$(echo "$dir" | sed "s/netdev2/netdev/g")"
                git mv -f "$dir" "$new_dir"
        done
git commit --amend
git ls-tree --full-tree -r HEAD --name-only | \
        grep "netdev2" | while read file; do
                new_file="$(echo "$file" | sed "s/netdev2/netdev/g")"
                git mv -f "$file" "$new_file"
        done
git commit --amend
git grep --name-only "\<drivers_netdev_netdev\>" | \
        xargs sed -i "s/\<drivers_netdev_netdev\>/drivers_netdev_api/g"
git add -p
git commit --amend
```

`res` is used to set the frame header iovec's length afterwards so overriding
it with a negative number is kind of a bad idea ;-).