O has no user-specified value, then O's default value is treated the same
way. If the default value still does not match, parse_enum() returns 0.
- Example: parse_enum (o, "a", 1, "b", 2, (char *) NULL) returns 1 if O's
+ Example: parse_enum (o, "a", 1, "b", 2, NULL_SENTINEL) returns 1 if O's
value if "a", 2 if O's value is "b".
Destroys O. */