systemctl: fail in the case that no unit files were found

Previously systemctl died with message

-bash-4.2# systemctl --root /rawhi list-unit-files
(src/systemctl/systemctl.c:868) Out of memory.

in the case that no unit files were found in the --root
or the directory did not exist.

So lets return ENOENT in the case that --root does not exist
and empty list in the case that there are no unit files.
build: remove repeated KMOD section
machine-id-setup: don't try to read UUID from VM/container manager if we operate on a root directory that's not /

This should make sure no UUID from the host systemd-machine-id-setup is
running on leaks onto a disk image that is provisioned with the tool.
resolved-dns-rr: fix typo

a->rrsig.type_covered != a->rrsig.type_covered" is always false
regardless of the values of its operands because those operands are identical.
resolved: fix which return codes we check

Discovered by Lukas Nykryn
journal-remote: remove unreachable code
util: return after freeing all members of array
journal-upload: make sure that 'r' is initialized
resolved: write resolv.conf search - switch arguments

Found by Lukáš Nykrýn.
sd-event: add API to access epoll_fd

This is a prerequisite for integrating sd-event into an external
event loop.
journalctl: add "-t --identifier=STRING" option

This turns journalctl to the counterpart of systemd-cat.
Messages sent with

systemd-cat --identifier foo --prioritiy debug

can now be shown with

journalctl --identifier foo --prioritiy debug

"--identifier" is not merged with "--unit" to make a clear
distinction between syslog and systemd units.
syslog identifiers can be chosen freely by anyone.
CODING_STYLE: document that we don't break lines at 80ch
util: change return value of startswith() to non-const

This way we can use it on non-const strings, and don't end up with a
const'ified result.

This is similar to libc's strstr() which also takes a const string but
returns a non-const one.
util: simplify close_nointr() a bit
util: make asynchronous_close() really work like an asynchronous version of safe_close()

Save/restore errno, like we do in safe_close(). And don't fork a thread
if the parameter is already negative.
core: unify how we generate the prefix string when dumping unit state
service: asynchronous_close() already checks for negative parameters, no need to duplicate that
service: remove some pointless linebreaks, to make things more readable
service: don't invoke functions at the same time as declaring variables
service,strv: introduce strv_find_startswith() and make use of it

Unlike strv_find_prefix() the new call will return a pointer to the
suffix of the item we found, instead of the whole item. This is more
closer inline with what startswith() does, and allows us to simplify a
couple of invocations.