condition: use 'selinux' rather than 'SELinux' as preferred spelling

The virtualization condition and others use lowercase identifiers, so
for the sake of keeping things least surprising, use lowercase
identifiers here too.

man/systemd.unit.xml

@@ -634,7 +634,7 @@
                                 may be used to check whether the given security
                                 module is enabled on the system.
                                 Currently the only recognized value is
-                                <varname>SELinux</varname>.
+                                <varname>selinux</varname>.
                                 The test may be negated by prepending an
                                 exclamation mark. Finally,
                                 <varname>ConditionNull=</varname> may

src/condition.c

@@ -134,7 +134,7 @@ static bool test_virtualization(const char *parameter) {
 
 static bool test_security(const char *parameter) {
 #ifdef HAVE_SELINUX
-        if (!strcasecmp(parameter, "SELinux"))
+        if (streq(parameter, "selinux"))
                 return is_selinux_enabled() > 0;
 #endif
         return false;