Rename and export MaxScaleUptime()

MaxScaleUptime() renamed to maxscale_uptime() and moved from
gateway.c (MaxScale main) to misc.c, which is included in the
maxscale_common library. That way the symbol will be available
at link-time and will thus not prevent the use of the linker
flags -Wl,-z,defs (resolve all symbols at link-time) when
linking a module that uses maxscale_uptime().

server/include/maxscale.h

@@ -40,4 +40,8 @@
 #define MAXSCALE_ALREADYRUNNING	4	/* MaxScale is already runing */
 #define MAXSCALE_BADARG		5	/* Bad command line argument */
 #define MAXSCALE_INTERNALERROR	6	/* Internal error, see error log */
+
+void maxscale_reset_uptime(void);
+int maxscale_uptime(void);
+
 #endif