* 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
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