[Libguestfs] [PATCH nbdkit 2/2] server: Avoid modifying argv by saving keys in a list and freeing on exit.

Unfortunately you cannot restore argv by setting *p = '=' :-( The reason is we advertize that plugins are allowed to save they ‘const char *key’ pointer passed to them in .config, but assigning *p = '=' changes the key string from "key" back to "key=value". Surprisingly only test-eval.sh actually broke, but other plugins are undoubtedly affected. My alternate fix is fairly horrible, but passes all the tests and valgrind. Rich.