Move preload_dso() to its own file and rename to sudo_preload_dso().

It now takes an intercept fd as an optional argument instead of a
list of extra variables to add.  This lets us check whether it is
already set to the expected value (and add it if not).
sudo_intercept.so now uses sudo_preload_dso() to make sure that
LD_PRELOAD and SUDO_INTERCEPT_FD are set properly before executing.

src/sudo_exec.h

@@ -117,4 +117,7 @@ bool utmp_login(const char *from_line, const char *to_line, int ttyfd,
     const char *user);
 bool utmp_logout(const char *line, int status);
 
+/* exec_preload.c */
+char **sudo_preload_dso(char *envp[], const char *dso_file, int intercept_fd);
+
 #endif /* SUDO_EXEC_H */