Add dynamic modules on Posix.

You can now add `DYNAMIC` as an option to `px4_add_module`, which will
cause that module to no longer be compiled into the px4 executable, but
instead produce a separate shared library file, which can be loaded and
executed with the new `dyn` command:

    pxh> dyn ./hello.px4mod start

This will load the shared object file `hello.px4mod` if it wasn't
already loaded, and execute its main function with the given arguments.

src/systemcmds/dyn/CMakeLists.txt

@@ -0,0 +1,8 @@
+px4_add_module(
+	MODULE systemcmds__dyn
+	MAIN dyn
+	SRCS
+		dyn.cpp
+	)
+
+target_link_libraries(systemcmds__dyn PUBLIC dl)

src/systemcmds/dyn/dyn.cpp

@@ -0,0 +1,91 @@
+/****************************************************************************
+ *
+ *   Copyright (C) 2018 PX4 Development Team. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ * 3. Neither the name PX4 nor the names of its contributors may be
+ *    used to endorse or promote products derived from this software
+ *    without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
+ * ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ *
+ ****************************************************************************/
+
+/**
+ * @file dyn.cpp
+ *
+ * @author Mara Bos <m-ou.se@m-ou.se>
+ */
+
+#include <dlfcn.h>
+
+#include <px4_module.h>
+#include <px4_log.h>
+
+static void usage();
+
+extern "C" {
+	__EXPORT int dyn_main(int argc, char *argv[]);
+}
+
+static void usage()
+{
+	PRINT_MODULE_DESCRIPTION(
+		R"(
+### Description
+Load and run a dynamic PX4 module, which was not compiled into the PX4 binary.
+
+### Example
+$ dyn ./hello.px4mod start
+
+)");
+	PRINT_MODULE_USAGE_NAME_SIMPLE("dyn", "command");
+	PRINT_MODULE_USAGE_ARG("<file>", "File containing the module", false);
+	PRINT_MODULE_USAGE_ARG("arguments...", "Arguments to the module", true);
+}
+
+int dyn_main(int argc, char *argv[]) {
+	if (argc < 2) {
+		usage();
+		return 1;
+	}
+
+	void *handle = dlopen(argv[1], RTLD_NOW);
+
+	if (!handle) {
+		PX4_ERR("%s", dlerror());
+		return 1;
+	}
+
+	void *main_address = dlsym(handle, "px4_module_main");
+
+	if (!main_address) {
+		PX4_ERR("%s", dlerror());
+		dlclose(handle);
+		return 1;
+	}
+
+	auto main_function = (int (*)(int, char **))main_address;
+
+	return main_function(argc - 1, argv + 1);
+}