sd-rtnl: minor fixes
path_check_timestamp: only keep the most recent timestamp

There is no point in keeping one timestamp for each directory, as we only
ever care about the most recent one.
udev: link-config - use _cleanup_ macro locally
rtnl: use _cleanup_ macro more
rtnl: use malloc0 instead of calloc
udev: link-config - use new0 instead of calloc
udev: link-config - use proper return values

Not sure if -ENOENT is the correct return value for when no persistent network
name is set, but couldn't think of anything better.
udev: link-config - use zero instead of memset
udev: link-config - use safe_atou instead of strtoul
rtnl: introduce default timeout

We set it to 10 secs (as we are only communicating with the kernel,
it seems we should be able to bail out sooner than sd-bus, which
uses 25).

When passing timout 0, the default is used, use this in link-config.
rtnl: complain if used after fork
udev: link-config - split connection to sockets from loading of configs

We want to load the config in _init, but not connect to the sockets before we are forked.
rtnl: cleanup socket_read_message
bus: add new sd_bus_message_verify_type() and sd_bus_message_copy() API calls
bus: add a macro to simplify reading of UUIDs
timedate: handle more nicely if something or somebody keeps open /dev/rtc and thus blocks out everybody else

chrony is appears to keep the RTC open continuously these days which is
a bad idea, and /dev/rtc is a single-user device, which is a bad idea
too. Together both bad ideas mean that nobody else can access the RTC
anymore. That's something to fix, but in the meantime we should handle
this more gracefully.
rc-local: use access(X_OK) to check if files are executable

... and other modernizations.
strv: introduce new strv_from_stdarg_alloca() macro to generate a string array from stdarg function parameters

This allows us to turn lists of strings passed in easily into string
arrays without having to allocate memory.
bus: add extra validity checks
strv: introduce new STRV_MAKE and STRV_MAKE_EMPTY macros to create string arrays nicely on the fly