core: introduce systemd.early_core_pattern= kernel cmdline option

core: introduce systemd.early_core_pattern= kernel cmdline option

Until a core dump handler is installed by systemd-sysctl, the generation of
core dump for services is turned OFF which can make the debugging of the early
boot process harder especially since there's no easy way to restore the core
dump generation.

This patch introduces a new kernel command line option which specifies an
absolute path where the kernel should write the core dump file when an early
process crashes.

This will take effect until systemd-coredump (or any other handlers) takes
over.