conf: Replace hard-coded /etc where appropriate

Also document the actual value of ${sysconfdir}.

conf/options/charon.opt

@@ -38,8 +38,8 @@ charon.cert_cache = yes
 charon.cache_crls = no
 	Whether Certificate Revocation Lists (CRLs) fetched via HTTP or LDAP should
 	be saved under a unique file name derived from the public key of the
-	Certification Authority (CA) to **/etc/ipsec.d/crls** (stroke) or
-	**/etc/swanctl/x509crl** (vici), respectively.
+	Certification Authority (CA) to **${sysconfdir}/ipsec.d/crls** (stroke) or
+	**${sysconfdir}/swanctl/x509crl** (vici), respectively.
 
 charon.check_current_path = no
 	Whether to use DPD to check if the current path still works after any