man: don't document systemctl --failed

This effectively reverts 599b6322f19ec619ddc294d0e7880b669040cf66, which
in turn partially reverted 4dc5b821ae737914499119e29811fc3346e3d97c.

The --failed switch is not documented on purpose, since it is redundant
due to --state=failed, which it predates. Due to that it's not
documented in --help either.

We generally try to avoid redundant interfaces, but if we need to keep
them for compatibility we do so, however remove them from documentation
to ensure they are not used in future.

The man page is now changed to include a comment about the fact that
--failed is not documented on purpose. Also, explicitly mention
--state=failed as example for --state.
man: don't refer to undocumented option '--failed'
build-sys: add libcap to libsystemd-shared
build-sys: test-fdset - add libsystemd-internal
build-sys: do not use "label" functions in libsystemd-shared
inhibit: allow filtering --list by mode

Usually there are a few delay inhibitors all the time (NetworkManager,
Telepathy, etc.), but I'm only interested in the block ones.
busctl: add "tree" command to explore object trees
update TODO
shared/install: when unit contains only Also=, report 'indirect'

If a unit contains only Also=, with no Alias= or WantedBy=, it shouldn't
be reported as static. New 'indirect' status shall be introduced.

https://bugzilla.redhat.com/show_bug.cgi?id=864298
man: improve documentation for "indirect" unit file state a bit

Also, correct mentions of "units" instead of "unit files" in the table,
and terminate all sentences with a full stop.
shared: explicitly ignore the return value of wait_for_terminate

CID#1237532
CID#1237523
CID#1237522
bus: when dumping string property values escape the chars we use as end-of-line and end-of-item marks
strv: rework strv_split_quoted() to use unquote_first_word()

This should make the unquoting scheme a bit less naive.
update TODO
sd-bus: add sd_bus_message.verify_destination_id and .destination_ptr

kdbus learned to accept both a numerical destination ID as well as a
well-known-name. In that case, kdbus makes sure that the numerical ID is in
fact the owner of the provided name and fails otherwise.

This allows for race-free assertion of a bus name owner while sending a
message, which is a requirement for bus-proxyd.

Add two new fields to sd_bus_message, and set the numerical ID to
verify_destination_id if bus_message_setup_kmsg() is called for a
message with a well-known name.

Also, set the destination's name in the kdbus item to .destination_ptr
if it is non-NULL.

Normal users should not touch these fields, and they're not publicy
accessible.
bus-proxyd: keep track of names acquired by legacy client

Store names successfully acquired by the legacy client into a hashmap.
We need to take these names into account when checking for send policies.
bus-proxyd: enforce policy for Hello messages
bus-proxyd: enforce policy for name ownership
bus-proxyd: enforce policy for method calls
bus-proxyd: move name list iteration to policy users

We need to figure out which of the possible names satisfied the policy,
so we cannot do the iteration in check_policy_item() but have to leave it
to the users.

Test cases amended accordingly.