Fallback time option was updating request_time instead of fallback_time.

Resolves #561

* IPv4: Ignore DHCP state when building routes

As this will change periodically.
We only care if the state (address) has been added.

Fixes #440 thanks to JognSmit.

ldop itself cannot be non NULL as it points to the location.
but *ldop CAN be NULL.

Fixes #567.

As such, we cannot access dhcpcd_ctx so we can't use the free
route list.
Fixes #571.

Fixes #573.

* options: Don't assume vsio options have an argument

Should fix #577.

On ILP32 and LP64 systems, unsigned long is sufficient to round-trip
pointers, but on LLP64 systems it is too small, and on systems like
CHERI where pointers are capabilities, it does not propagate capability
metadata, and casting back to a pointer to yield a pointer that cannot
be dereferenced.

Instead, cast via uintptr_t to ensure we always use an integral type
that can losslessly round-trip pointers.

* privsep: Ensure we recv for real after a successful recv MSG_PEEK

Adjust the code flow so that the same errors would be caught
after the final recv.
This ensures we read what is really meant for us and not
something silly.

Return EBADMSG on recvmsg len mismatch.

Missing parentheses in IP_UDP_SIZE caused wrong computation of MSZ
(option57). When mtu is 1500, current MSZ value is 1488=1500-20+8, while
the correct value should be 1472=1500-(20+8).

Also added parentheses for BOOTP_MIN_MTU.

Fixes #584

Helps #566

Fixes #582.

When an interface (re-)gains carrier dhcpcd_handlecarrier() runs
dhcpcd_initstate() to kick off profile re-selection. Previously this used
args originally passed when starting the manager (ctx->argv).

However interfaces started via the manager control
interface (dhcpcd_initstate1() in dhcpcd_handleargs()) may be started with
different args.

For example if we start a manager with

    dhcpcd -M --inactive

and then start only IPv4 on an interface with

    dhcpcd -4 iface0

a subsequent CARRIER event will reset the interface to what amounts to
"default config + `-M --inactive`" which in this case will enable ipv6
also!

To fix this we keep a copy of the arguments used to start an interface in
the manager (dhcpcd_handleargs()) code path around around (ifp->argv).

In the current implementation args passed for renew following the initial
interface start will not be persisted. This causes the interface to reset
to a state of "defaults + config + profile + start-cmdline".

For example (continuing the scenario above) after enabling ipv6 with -n:

    $ dhcpcd -6 -n iface0

A subsequent CARRIER event will disable ipv6 again as the effective
arguments remain `-4 iface0` as passed during interface start.

Note the per-interface daemon code path wasn't affected as ctx->args
already contains the interface start args.

This will affect FreeBSD and OpenBSD.
Use sendmsg to send the length of the interface name, then the
interface name and then the data rather than just sending
IFNAMSIZ which may have uninitialised bytes at the end.

Opimise ps_sendcmdmsg while here.
Hopefully helps #565 but I'm not hopeful.