'RELENG_5_3_0_RELEASE'.

This commit was manufactured to restore the state of the 5.3-RELEASE image.

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

re-breaks non-interactive portupgrade (or at least old versions of
portupgrade); I'll see if I can put together a solution which avoids
breaking anything later.

Approved by:	rwatson (mentor)
Noticed by:	Stefan Farfeleder, Joshua Goodall

Sync usage with manpage.

1. Don't do tty stuff to stdin if stdin isn't a tty.
2. When running in non-interactive mode, don't select(2)
on the standard input.

This un-breaks non-interactive portupgrade.

PR: bin/59036 [1]
PR: bin/56166, bin/57414, ports/57415, ports/60534 [2]
MFC after: 7 days
Approved by: rwatson (mentor)

Add some constness to avoid some warnings.
Remove use register keyword.
Deal with missing/unneeded extern/prototypes.
Some minor type changes/casts to avoid warnings.

Reviewed by:	md5

deprecated 4.2/4.3BSD wait union.  Fix some nearby pid_t/int
confusion.