use child_process members "args" and "env" directly

use child_process members "args" and "env" directly

Build argument list and environment of child processes by using
struct child_process and populating its members "args" and "env"
directly instead of maintaining separate strvecs and letting
run_command_v_opt() and friends populate these members.  This is
simpler, shorter and slightly more efficient.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Taylor Blau <me@ttaylorr.com>