coredump: rely on /proc exclusively to get the name of the crashing process

coredump: rely on /proc exclusively to get the name of the crashing process

I couldn't see any reason why the kernel could provide COMM to the coredump
handler via the core_pattern command line but could not make it available in
/proc. So let's assume that this info is always available in /proc.

For "backtrace" mode (when --backtrace option is passed), I assumed that the
crashing process still exists at the time systemd-coredump is called.

Also changing the core_pattern line is an API breakage for any users of the
backtrace mode but given that systemd-coredump is installed in
/usr/lib/systemd, it's a private tool which has no internal users. At least no
one complained when the hostname was added to the core_pattern line
(f45b8015513)...

Indeed it's much easier to get it from /proc since the kernel substitutes '%e'
specifier with multiple strings if the process name contains spaces (!).