MFC r315005: sh: Fix executing wrong command with ${x#$(y)}$(z).

The parsed internal representation of words consists of a byte string with a
list of nodes (commands in command substitution). Each unescaped CTLBACKQ or
CTLBACKQ | CTLQUOTE byte corresponds to an entry in the list.

If param in ${param#%##%%word} is not set, the word is not expanded (in a
deviation of POSIX shared with other ash variants and ksh93). Erroneously,
the pointer in the list of commands (argbackq) was not advanced. This caused
the wrong command to be executed later if the outer word contained another
command substitution.

Example:
echo "${unsetvar#$(echo a)}$(echo b)"
wrote "a" but should write "b".

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

MFC r314637: sh: Add some already working tests that exercise new code paths

MFC r269902,r270101:

r269902:

Convert bin/sh/tests to ATF

The new code uses a "test discovery mechanism" to determine
what tests are available for execution

The test shell can be specified via:

kyua test -v test_suites.FreeBSD.bin.sh.test_shell=/path/to/test/sh

Sponsored by: EMC / Isilon Storage Division
Approved by: jmmv (mentor)
Reviewed by: jilles (maintainer)

r270101 (by jilles):

sh: Don't hardcode relative paths in the tests stderr files.

These paths have had to be adjusted to changes in the testsuite runner
several times, so modify the tests to remove the need for such adjustment.

A cp in functional_test.sh is now unneeded, but this matters little in
performance.

MFC r268436,273243,278806,280920,286971: sh: Add various tests.

These new tests from head pass with unmodified stable/10 sh.

MFC r273920: sh: Fix corruption of CTL* bytes in positional parameters in
redirection.

EXP_REDIR was not being checked for while expanding positional parameters in
redirection, so CTL* bytes were not being prefixed where they should be.

MFC r268436,273243,278806,280920,286971: sh: Add various tests.

These new tests from head pass with unmodified stable/10 sh.

MFC r273920: sh: Fix corruption of CTL* bytes in positional parameters in
redirection.

EXP_REDIR was not being checked for while expanding positional parameters in
redirection, so CTL* bytes were not being prefixed where they should be.