Added configurable executable file directory

This allows MaxScale to launch processes from the directory where
the `maxscale` executable is located.

server/core/gwdirs.c

@@ -96,6 +96,18 @@ void set_libdir(char* param)
     libdir = param;
 }
 
+/**
+ * Set the executable directory. Internal processes will look for executables
+ * from here.
+ * @param str Path to directory
+ */
+void set_execdir(char* param)
+{
+    free(execdir);
+    clean_up_pathname(param);
+    execdir = param;
+}
+
 /**
  * Get the directory with all the modules.
  * @return The module directory
@@ -158,3 +170,12 @@ char* get_langdir()
 {
     return langdir ? langdir : (char*) default_langdir;
 }
+
+/**
+ * Get the directory with the executables.
+ * @return The executables directory
+ */
+char* get_execdir()
+{
+    return execdir ? execdir : (char*) default_execdir;
+}