MFC r314686: sh: Fix crash if a -T trap is taken during command substitution.

MFC r314686: sh: Fix crash if a -T trap is taken during command substitution.

Code like  t=$(stat -f %m "$file")  segfaulted if -T was active and a trap
was taken while the shell was waiting for the child process to finish.

What happened was that the dotrap() call in waitforjob() was hit. This
re-entered command execution (including expand.c) at a point not expected by
expbackq(), and global state (unallocated stack string and argbackq) was
corrupted.

To fix this, change expbackq() to prepare for command execution to be
re-entered.

In stable/10, there is more global state that needs to be restored than in
stable/11 and head.

Reported by:	bdrewery

git-svn-id: svn://svn.freebsd.org/base/stable/10@316942 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f