Decouple "Unit" from NXT_SERVER.

Split out the "Unit" name from the NXT_SERVER #define into its own
NXT_NAME #define, then make NXT_SERVER a combination of that and
NXT_VERSION.

This is required for a subsequent commit where we may want the server
name on its own.

Reviewed-by: Alejandro Colomar <alx@nginx.com>
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

src/nxt_main.h

@@ -11,7 +11,8 @@
 #include <nxt_auto_config.h>
 #include <nxt_version.h>
 
-#define NXT_SERVER                   "Unit/" NXT_VERSION
+#define NXT_NAME                     "Unit"
+#define NXT_SERVER                   NXT_NAME "/" NXT_VERSION
 
 typedef struct nxt_port_s            nxt_port_t;
 typedef struct nxt_task_s            nxt_task_t;