cryptsetup-generator: set high OOM score for systemd-cryptsetup instances

With new LUKS2 header format it is possible to use Argon2 key derivation
function. This function is "memory-hard" hence keyslot unlocking can
potentially use a lot of RAM as this increases resistance to massively
parallel GPU based password cracking.

However, when multiple systemd-cryptsetup binaries run at the same
time it is very likely that system using Argon2 (e.g. Fedora 30)
will encounter memory-pressure during early boot, following OOM killing
spree.

This patch aims to lower the damage done by OOM killer and sets OOMScore
for systemd-cryptsetup units to 500. Hopefully OOM killer will then
shoot us down and leave rest of the system services alone.

src/cryptsetup/cryptsetup-generator.c

@@ -265,6 +265,7 @@ static int create_disk(
                 "RemainAfterExit=yes\n"
                 "TimeoutSec=0\n" /* the binary handles timeouts anyway */
                 "KeyringMode=shared\n" /* make sure we can share cached keys among instances */
+                "OOMScoreAdjust=500\n" /* unlocking can allocate a lot of memory if Argon2 is used */
                 "ExecStart=" SYSTEMD_CRYPTSETUP_PATH " attach '%s' '%s' '%s' '%s'\n"
                 "ExecStop=" SYSTEMD_CRYPTSETUP_PATH " detach '%s'\n",
                 name_escaped, u_escaped, strempty(password_escaped), strempty(filtered_escaped),