Allow space between Defaults@ and hostname list

[squell]

Closes #1508

Needs regression tests in sudoers/test/mod.rs to also check edge cases such as:

Defaults@\
  ALL lecture

are accepted (edit ogsudo rejects these too) and that

Defaults@# a comment ALL lecture

and

Defaults@# a comment
ALL lecture

and

Defaults@# a comment \
ALL lecture

are rejected

[squell]

Note: commit 516334e is debatable; ogsudo doesn't allow line continuations at all in host/user/...-specific defaults (it would be interesting to look at how their parser solves the conundrum). We used to not allow it only in @ because of the stream.get_pos().0 == begin_pos.0 check (which checks we are still on the same line), but for Defaults@ it's superfluous.

During early sudo-rs development I've often had discussions with @rnijveld where he expressed severe doubts if we could manage to parse /etc/sudoers with a relatively textbook look-ahead-1 recursive descent parser. Turns out that we still can.. more or less. (Maybe you could argue resetting the parsing position is definitely a form of backtracking; but I think it's an implementation trick only... maybe; this would be a nice topic for a bachelor's thesis in theoretical computer science)

[squell]

Note: b294cba can add the strictness here that ogsudo has (but it introduces a little bit extra complexity to a delicate part). I think if we're going to go one way or the other for Defaults@ it should at least be consistent.