add margo_init_opt() variant to pass Mercury opts

- proposed by Tommaso Tocci 
- follows the naming convention of the Mercury API
- retains existing margo_init() for compatibility

include/margo.h

@@ -67,12 +67,43 @@ typedef ABT_eventual margo_request;
  * call margo_wait_for_finalize() after margo_init() to relinguish control to 
  * Margo.
  */
-margo_instance_id margo_init(
+#define margo_init(_addr_str, _mode, _use_progress_thread, _rpc_thread_count)\
+ margo_init_opt(_addr_str, _mode, NULL, _use_progress_thread, _rpc_thread_count)
+
+/**
+ * Initializes margo library with custom Mercury options.
+ * @param [in] addr_str            Mercury host address with port number
+ * @param [in] mode                Mode to run Margo in:
+ *                                     - MARGO_CLIENT_MODE
+ *                                     - MARGO_SERVER_MODE
+ * @param [in] hg_init_info        (Optional) Hg init info, passed directly
+ *                                 to Mercury
+ * @param [in] use_progress_thread Boolean flag to use a dedicated thread for
+ *                                 running Mercury's progress loop. If false,
+ *                                 it will run in the caller's thread context.
+ * @param [in] rpc_thread_count    Number of threads to use for running RPC
+ *                                 calls. A value of 0 directs Margo to execute
+ *                                 RPCs in the caller's thread context.
+ *                                 Clients (i.e processes that will *not* 
+ *                                 service incoming RPCs) should use a value 
+ *                                 of 0. A value of -1 directs Margo to use 
+ *                                 the same execution context as that used 
+ *                                 for Mercury progress.
+ * @returns margo instance id on success, MARGO_INSTANCE_NULL upon error
+ *
+ * NOTE: Servers (processes expecting to service incoming RPC requests) must
+ * specify non-zero values for use_progress_thread and rpc_thread_count *or*
+ * call margo_wait_for_finalize() after margo_init() to relinguish control to 
+ * Margo.
+ */
+margo_instance_id margo_init_opt(
     const char *addr_str,
     int mode,
+    const struct hg_init_info *hg_init_info,
     int use_progress_thread,
     int rpc_thread_count);
 
+
 /**
  * Initializes margo library from given argobots and Mercury instances.
  * @param [in] progress_pool Argobots pool to drive communication

src/margo.c

@@ -171,7 +171,7 @@ static hg_return_t margo_handle_cache_put(margo_instance_id mid,
 static hg_id_t margo_register_internal(margo_instance_id mid, hg_id_t id,
     hg_proc_cb_t in_proc_cb, hg_proc_cb_t out_proc_cb, hg_rpc_cb_t rpc_cb, ABT_pool pool);
 
-margo_instance_id margo_init(const char *addr_str, int mode,
+margo_instance_id margo_init_opt(const char *addr_str, int mode, const struct hg_init_info *hg_init_info,
     int use_progress_thread, int rpc_thread_count)
 {
     ABT_xstream progress_xstream = ABT_XSTREAM_NULL;
@@ -268,7 +268,7 @@ margo_instance_id margo_init(const char *addr_str, int mode,
         rpc_pool = progress_pool;
     }
 
-    hg_class = HG_Init(addr_str, listen_flag);
+    hg_class = HG_Init_opt(addr_str, listen_flag, hg_init_info);
     if(!hg_class) goto err;
 
     hg_context = HG_Context_create(hg_class);