fix: suppress prompt setup in screen restore

[ethanpailes]

This patch starts correctly suppressing the prompt setup code during screen restore. Previously, we were just suppressing it in terms of the data we initially stream to the user, but if you create a new session, then disconnect, then reconnect, the setup code would be visible. The fix is to avoid pumping data into the in-memory term until after we've seen the prompt setup sentinel.

I tested that the tweaked test in this patch failes without the application change to ensure that it is really testing what we want. I also observed this patch working with manual testing.

Fixes #120

[Aetf]

error: this expression creates a reference which is immediately dereferenced by the compiler
   --> shpool/tests/support/line_matcher.rs:173:36
    |
173 |             if nomatch_re.is_match(&line) {
    |                                    ^^^^^ help: change this to: `line`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow
    = note: `-D clippy::needless-borrow` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::needless_borrow)]`