Attempt #2 to fix script(1) if the standard input is closed: If we

Attempt #2 to fix script(1) if the standard input is closed: If we
read EOF from STDIN_FILENO, write zero bytes into the pseudo-terminal;
this is interpreted as an EOF by the program being scripted.

I've tested this with two non-interactive scripts:
# echo 5 | script foo sh -c 'read x; sleep $x; echo bar'
# echo bar | xargs script foo echo
and one interactive program:
# script foo more /etc/passwd
and everything seems to work properly...

PR:		bin/56166, bin/57414, ports/57415, ports/60534
Approved by:	rwatson (mentor)
MFC after:	2 weeks