Implementing MG interfaces

PUBLISHED_FROM=46496c2d5340a0bbe4fe1c6c9ff187bc65d2d35f

docs/c-api/net.h/struct_mg_mgr_init_opts.md

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+---
+title: "struct mg_mgr_init_opts"
+decl_name: "struct mg_mgr_init_opts"
+symbol_kind: "struct"
+signature: |
+  struct mg_mgr_init_opts {
+    struct mg_iface_vtable *main_iface;
+    int num_ifaces;
+    struct mg_iface_vtable **ifaces;
+  };
+---
+
+Optional parameters to `mg_mgr_init_opt()`.
+
+If `main_iface` is not NULL, it will be used as the main interface in the
+default interface set. The pointer will be free'd by `mg_mgr_free`.
+Otherwise, the main interface will be autodetected based on the current
+platform.
+
+If `num_ifaces` is 0 and `ifaces` is NULL, the default interface set will be
+used.
+This is an advanced option, as it requires you to construct a full interface
+set, including special networking interfaces required by some optional
+features such as TCP tunneling. Memory backing `ifaces` and each of the
+`num_ifaces` pointers it contains will be reclaimed by `mg_mgr_free`. 
+