WIP: Avoid Expect clobbering stdout when stdin isn't a tty.

Doing expect_after before a child process has been spawned causes Expect
to try reading from stdin. If stdin is /dev/null, it gets an EOF from
the read, assumes it's been closed... and closes stdout too, so we don't
see any more of the child process interaction.

WIP because I've only done this for simh.

build/build.tcl

@@ -106,7 +106,11 @@ proc build_macsyma_portion {} {
 }
 
 set timeout 100
-expect_after timeout abort
+proc setup_timeout {} {
+    # Don't do this until after you've called "spawn", otherwise it'll cause a
+    # read from stdin which will return EOF if stdin isn't a tty.
+    expect_after timeout abort
+}
 
 set ip [ip_address [lindex $argv 0]]
 set gw [ip_address [lindex $argv 1]]

build/simh/build.tcl

@@ -4,6 +4,8 @@ set emulator_escape "\034"
 
 proc start_salv {} {
     uplevel #0 {spawn pdp10 build/simh/init}
+    setup_timeout
+
     respond "sim>" "show ver\r"
     respond "sim>" "b tu1\r"
     expect "MTBOOT"
@@ -23,6 +25,7 @@ proc start_dskdmp {} {
 
 proc start_its {} {
     uplevel #0 {spawn pdp10 build/simh/boot}
+    setup_timeout
 }
 
 proc mount_tape {file} {