vmcore: don't fail if /etc/kdump.conf is not readable

Signed-off-by: Richard Marko <rmarko@redhat.com>
vmcore: use re.MULTILINE instead of numerical value

Signed-off-by: Richard Marko <rmarko@redhat.com>
spec: vmcore: require kexec-tools

Signed-off-by: Richard Marko <rmarko@redhat.com>
vmcore: fail gracefully if dump_dir is not accessible

Previously os.listdir on inaccessible dump_dir resulted
in exception. This replaces insufficient os.path.exists check
(which is already handled in abrt-vmcore.service file) with
os.access call which checks for dir readability.

Also adding proper error handling for shutil calls later on.

Signed-off-by: Richard Marko <rmarko@redhat.com>