sys/cmd/procsubst: work around hang on AT&T ksh93: fork subshell
sys/cmd/procsubst: work around hang on AT&T ksh93: fork subshell

Yet another problem with NONFORKSUBSH avoided by forcing a fork of
the current subshell. Since '%' is expected to be run in a command
substitution, we can force a fork using a dummy redirection 1>&1.
(Outside of a command substitution, the hanging problem doesn't
occur anyway.)

Also, comments edit: stop recommending 'eval'. (re: fafbb708)

modernish/modernish