Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFC r226892,r228007,r228873,r230121,r232839: sh: Various testcases that
already work.

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

sh: Do parameter expansion on ENV before using it.

This is required by POSIX, and allows things like ENV=\$HOME/.shrc.

Note that tilde expansion is explicitly not performed.

sh: Add test for positional parameters with more than one digit.

This also passes on stable/8.

Allow one to regression test 'sh' changes without having to install
a potentially bad /bin/sh first.

sh: On startup of the shell, use PWD from the environment if it is valid.
Unset PWD if it is incorrect and no value for it can be determined.
This preserves the logical current directory across shell invocations.

Example (assuming /home is a symlink):
$ cd
$ pwd
/home/foo
$ sh
$ pwd
/home/foo

Formerly the second pwd would show the physical path (symlinks resolved).

sh: Test that bogus values of PWD are not imported from the environment.

Current versions pass this test trivially by never importing PWD, but I plan
to change sh to import PWD if it is an absolute pathname for the current
directory, possibly containing symlinks.

sh: Make sure the mail2.0 test can actually fail if $MAIL is not touched.

sh: Do not stat() $MAIL/$MAILPATH in non-interactive shells.

These may be NFS mounted, and we should not touch them unless we are going
to do something useful with the information.

Add a test for r185231.