net-util: add support for Type=ethernet

When DEVTYPE is not set for a nic, it means it is a wired/ethernet
device.
man: networkd - document DHCP option
man: document a batch of bus functions
systemctl: improve readability on failed commands

Not long ago a failed command would print:
"Failed to start something.service: ..."
regardless of whether the command was to start/stop/restart/etc.

With e3e0314 this was improved to print the method used. E.g. for stopping:
"Failed to StopUnit something.service: ..."

This patch matches the method to a more human readable word. E.g:
"Failed to stop something.service: ..."
Revert "net-util: add support for Type=ethernet"

This reverts commit 4cd1214db6cf4b262e8ce6381bc710091b375c96.

This may still be fixed in the kernel, revert this for now until
we see how it all shakes out.
udev: net_id - handle virtio buses

This was already supported in path_id, so should be uncontroversial.
sd-event: add missing symbol, sd_event_source_set_io_fd

This appears to have been omitted in 30caf8f3afd29da8 when the
the function was added.
man: typo fix
networkd: refuse to use .network files with missing Address/Gateway key

These keys are mandatory in [Address]/[Route] sections. Otherwise, we
hit an assert:

ens3: setting addresses
Assertion 'address->family == 2 || address->family == 10' failed at /build/amd64-generic/tmp/portage/sys-apps/systemd-9999-r1/work/systemd-9999/src/network/networkd-address.c:137, function address_configure(). Aborting.

Reported-by: Alex Polvi <alex.polvi@coreos.com>

At the same time make sure Route's Destination and Gateway uses the same address family.
man: document some more bus functions
swap: remove if/else with the same data path

This was introduced in e1770af812 (2012-02-03, swap: replace failure
boolean by result enum).

This just removes unneeded lines of code, no functional change.
Update .mailmap file

This commit updates email addresses of people, who are already in the
.mailmap file, so I'd assume they have sorted out their viewpoint on
privacy within the .mailmap file.

The entries for this commit have been produced using:
        # Finding out duplicates by comparing email addresses:
        git shortlog -sne |awk '{ print $NF }' |sort |uniq -d

        # Finding out duplicates by comparing names:
        git shortlog -sne |awk '{ NF--; $1=""; print }' |sort |uniq -d
shared: procfs_file_alloca: handle pid==0

when pid is set to 0 use /proc/self
util.c: use read_one_line_file where possible
bus: open user, not system, bus in bus_open_user_systemd()
bus: driverd: don't attempt to remove from empty list
hwdb: update
TODO: extend login capability note

We cannot remove CAP_SYS_ADMIN, which basically makes removing all other
capabilities useless. Anyhow, still wouldn't hurt checking whether stuff
like CAP_KILL can be dropped from logind.
shared: util.c: unify split and split_quoted
strv: multiple cleanups

- turn strv_merge into strv_extend_strv.
   appending strv b to the end of strv a instead of creating a new strv
- strv_append: remove in favor of strv_extend and strv_push.
- strv_remove: write slightly more elegant
- strv_remove_prefix: remove unused function
- strv_overlap: use strv_contains
- strv_printf: STRV_FOREACH handles NULL correctly