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

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

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

Approved by: re (implicit)

Eliminate more dead stores.

Found by: Clang static analyzer
MFC after: 7 days

wordexp(3): fix some bugs with signals and long outputs
* retry various system calls on EINTR
* retry the rest after a short read (common if there is more than about 1K
of output)
* block SIGCHLD like system(3) does (note that this does not and cannot
work fully in threaded programs, they will need to be careful with wait
functions)

PR: 90580
MFC after: 1 month

Fix typo: WRDE_DOOFS -> WRDE_DOOFFS.

Noticed by: Stoned Elipot

Terminate execl()'s argument list with a null pointer instead of a
null pointer constant. (The latter may be an integer constant, which
is not correct here.)

Submitted by: Stefan Farfeleder

Use hidden names (_close, _dup2, _waitpid, etc.) where appropriate.

Fix a number of bugs noticed by more extensive testing:
o Call waitpid() if an error occurs after forking the child process
to avoid leaving zombies around.
o Handle the WRDE_DOOFS|WRDE_APPEND combination correctly
o Do not confuse $( substitution with $(( shell arithmetic
(noticed by wollman)
o Handle backslash escaping properly
o Allow $( and ${ to be quoted

Be more consistent with "static".

Add an implementation of the POSIX wordexp() and wordfree() functions,
which perform shell-style word expansion on strings. This is still a
little rough around the edges.

PR: 13420