conf: add missing passenv to tty blocks

The line-based format has had the flag since v4.4 (issue #286), where
it prepends -p to the built-in getty, which turns it into login -p and
passes the environment on.  The block format was written from the three
documented tty variants and the flags listed in the tty documentation,
and passenv was in neither, so it was left out.  Converting a tty line
that used it therefore lost it, with nothing said.

It only reaches the built-in getty.  An external getty is handed its
arguments through command, so there is nowhere to put a -p, and the
setting is refused with a warning rather than quietly ignored.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

doc/ChangeLog.md

@@ -8,6 +8,8 @@ All relevant changes are documented in this file.
 
 ### Changes
 
+- The `tty` block takes `passenv`, which the line-based format has had
+  since v4.4 (issue #286) and the block format was missing
 - New `provides` setting for run/task/service/sysv blocks, naming
   conditions the service asserts in addition to its own `pid/<ident>`.
   Lets variants of one service, qualified with `if`, share the barrier

doc/config/migration.md

@@ -216,6 +216,14 @@ become
         notty    = true
     }
 
+The bare flags keep their names as booleans: `noclear`, `nowait`,
+`nologin`, `passenv`, `rescue`, e.g. `passenv` becomes
+`passenv = true`.
+
+A `tty` block is named by the device, not the title, so all three above
+show up as `tty:ttyS0` and the like in `initctl`.  The title still has
+to be unique within the file.
+
 Top-level directives
 --------------------
 

doc/config/tty.md

@@ -24,6 +24,7 @@ Settings common to all three:
 | `noclear` | | Do not clear the TTY after each session |
 | `nowait` | | Do not wait for Enter before the login prompt |
 | `nologin` | | Skip login, give a shell straight away |
+| `passenv` | | Pass the environment on to `login`, built-in getty only |
 
 The `device` variant takes two more: