util: make use of the new getrandom() syscall if it is available when needing entropy

Doesn't require an fd, and could be a bit faster, so let's make use of
it, if it is available.
missing: simplify memfd ifdeffery
update TODO
missing: no tabs please, we are british
sd-bus: properly handle removals of non-existing matches
nspawn: ignore EEXIST when creating mount point

A combination of commits f3c80515c and 79d80fc14 cause nspawn to
silently fail with a commandline such as:

  # systemd-nspawn -D /build/extra-x86_64 --bind=/usr

strace shows the culprit:

  [pid 27868] writev(2, [{"Failed to create mount point /build/extra-x86_64/usr: File exists", 82}, {"\n", 1}], 2) = 83
busctl: add new "capture" verb to record bus messages in libpcap compatible files, for dissection with wireshark
udev: path_id - update comments
missing.h: fix wrong __NR_getrandom syscall def

278 is vmsplice on x86_64. 318 is what we want:

http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/arch/x86/syscalls/syscall_64.tbl