This was primarily done so the backoff can be tested

Otherwise, the result might flow over on 8/16-bit platforms

Currently the constructed NA for a delayed NA case is neither used nor
released nor does it get an IPv6 header to be used properly. This fixes
that case.

When working on the previous commit I was unsure if a
garbage-collectible entry should remain in the list, so I added this
comment so I don't have to wonder about this in the future ;-).

Fragment size calculation previously failed for devices that are able to
transmit bigger layer 2 PDUs that 802.15.4 devices. This commit fixes the issue.

When the neighbor cache is full, the packet trying to resolve a
neighbor is not released.

This change fixes that.

The `_next_removable` list manages the cache-out of the neighbor cache.
However, when a neighbor cache entry is removed, it is not removed
from that list, which may lead to a segmentation fault when that list is
accessed, since the whole entry (including its list pointer) is zeroed
after removal.

With this change the entry is removed from that list accordingly before
the zeroing happens.

Added missing string for STATUS_COND_BLOCKED, so that the shell command "ps"
no longer crashes when a thread has this state.

Several parts were inspired by a driver written by:
  George Psimenos (gp7g14@soton.ac.uk)
  Steffen Robertz (steffen.robertz@rwth-aachen.de)

When either `gnrc_sixlowpan_iphc_nhc` or `gnrc_udp` is not compiled
in `_compressible()` never returns `true`. This causes the
`dispatch` snip in `gnrc_sixlowpan_iphc_send()` to be of length 0,
meaning `dispatch->data` is `NULL`, causing possible crashes when
trying to send IPv6 packets over 6LoWPAN without NHC or UDP.

Loosely based on [the original ping] and [netutil]'s ping

New features (compared to old RIOT version):
 - non-positional parameters
 - Better duplicate detection (addresses #9387)
 - Better asynchronous behavior
 - Potential for future move to `sock_ip`
 - (Optional) DNS-support
 - Multithreading-safe (in case shell-command handler gets called
   from multiple threads)

[the original ping]: http://ftp.arl.army.mil/~mike/ping.html
[netutil]: https://www.gnu.org/software/inetutils/

When IPv6 is enabled, the MTU is given. So users(*) sending IPv6 packets can
easily figure out what the supported maximum protocol unit is.

However, when IPv6 is disabled and a user wants to send layer 2 frames directly,
no information about the maximum PDU is available using the shell.

When 6LoWPAN is used, a user may be interested in the layer 2 PDU as well in
order to avoid layer 2 fragmentation.

This PR adds the L2-PDU info to the output of the ifconfig shell command, which
is printed regardless of the use of IPv6.

(*): Here "users" refers to human beings interacting with the shell.
Applications can get the maximum PDU of each layer more easily using
gnrc_netapi_get() with NETOPT_MAX_PACKET_SIZE instead of using a shell command.

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.

The shell handlers of the old, depreciated and removed LTC4150 driver are still
in place. This commit removes them

Currently an interface's existence is not checked when it is supplied
by the user with the `nib` command. This can lead to assertion errors
as soon as the generated entry tries to resolve an address or route
generated with that command and the network interface not being found.

Once the packet buffer is full on heavy network load, gnrc_netif_hdr_build may return NULL. In that case, the following unchecked access to hdr->data leads to a crash.

`gnrc_sixlowpan_frag` internally derives the offset value directly
from the fragment header, so for normal usage within GNRC this
assertion is redundant, but to make the tests of `rbuf_add` 100%
water-tide I added it.

Currently the loop just continues to run after a viable type is found.
In #10851 this lead to a crash of the tests, when the dependency of
`gnrc_sixlowpan` to `gnrc_ipv6` was removed.