Civi::pipe - Comply with newer `authx_login()` contract. Declare flow names.

Civi::pipe - Comply with newer `authx_login()` contract. Declare flow names.

1. The original Civi::pipe() patch was built on a draft version of `authx_login()`. The
   signature changed during review.
2. The original Civi::pipe() patch hacked the `flow` name - ie it used the
   flow name `xheader` even though it wasn't HTTP-based. Now, it
   uses either the existing `script` flow (for trusted/backend/principal-asserted runtimes)
   or the new `pipe` flow (for untrusted/credential-based runtimes).