Refactor code to set LD_PRELOAD (or the equivalent) in the environment

into a preload_dso() function.  Also avoid allocating a new copy
of the environment array if the size of the array does not change.

src/sudo_exec.h

@@ -74,7 +74,7 @@
 
 /* exec.c */
 struct sudo_event_base;
-int sudo_execve(const char *path, char *const argv[], char *const envp[], bool noexec);
+int sudo_execve(const char *path, char *const argv[], char *envp[], bool noexec);
 extern volatile pid_t cmnd_pid;
 
 /* exec_pty.c */