coredump: use "POSIX quotes" for cmdline

$ coredumpctl info |grep Command
  Command Line: bash -c kill -SEGV $$      (before)
  Command Line: bash -c "kill -SEGV \$\$"  (road not taken, C quotes)
  Command Line: bash -c $'kill -SEGV $$'   (now, POSIX quotes)

Before we wouldn't use any quoting, making it impossible to figure how the
command line was split into arguments. We could use "normal" quotes, but this
has the disadvantage that the commandline *looks* like it could be pasted into
the terminal and executed, but this is not true: various non-printable
characters cannot be expressed in this quoting style. (This is not visible in
this example). Thus, "POSIX quotes" are used, which should allow any command
line to be expressed acurrately and pasted directly into a shell prompt to
reexecute.

I wonder if we should another field in the coredump entry that simply shows the
original cmdline with embedded NULs, in the original /proc/*/cmdline
format. This would allow clients to format the data as they see fit. But I
think we'd want to keep the serialized form anyway, for backwards compatibility.