libsystemd: split up into subdirs

We still only produce on .so, but let's keep the sources separate to make things a bit
less messy.
bus: implement synchronous message calls via kernel ioctl
udev: static_node - do not exit rule after first static_node item

The nodes usually do not exist, so handle the next item instead of
skipping the entire rule.
bus: add API for querying the kdbus message sequence number
bus: simplify naming of feature negotation calls

Two verbs in a function name suck, so let's simplify this a bit.
bus: rename sd_bus_get_realtime_timestamp() to sd_bus_get_realtime_usec()

This brings the calls into similar style as the respective functions in
libsystemd-journal, and also is a bi shorter and more descriptive since
it clarifies the time unit used.
man: document message timestamping/sequence number API
bus: extend memfd api so that we can label memfds for debugging purposes
bus: add support for attaching name to bus connections for debugging purposes
bus: when allocating a memfd for usage in a bus connection, name the memfd after the bus connection
bus: include connection name in credentials structure
bus: pass timestamp info only to caller if he asked for it
sd-rtnl: link - allow setting the change mask
DEFINE_STRING_TABLE_LOOKUP: return _INVALID_* rather than assert on NULL string
sd-rtnl: add sd_rtnl_message_append_u16()
networkd: add basic bonding support

Refactor bridging support to be generic netdev support and extend it to
cover bonding as well.
sd-rtnl: simplify sd_rtnl_message_addr_new()

Split out into sd_rtnl_message_addr_set_{prefixlen,flags,scope}().
bus: unescape connection name when reading it from credentials
busctl: include connection name in busctl output
bus: set debug name of proxy bus connections