Added support for dynamic commands.

Dynamic commands are user commands that can be added dynamically to
the hub by a plugin.

The example plugin (mod_example.c) adds a !example command that when
invoked send a message to the user who invoked it.

src/adc/adcconst.h

@@ -20,7 +20,11 @@
 #ifndef HAVE_UHUB_ADC_CONSTANTS_H
 #define HAVE_UHUB_ADC_CONSTANTS_H
 
+#ifndef SID_T_DEFINED
 typedef uint32_t sid_t;
+#define SID_T_DEFINED
+#endif
+
 typedef uint32_t fourcc_t;
 
 /* Internal uhub limit */