* modules/shepherd/service.scm: add keep-tty flag to skip setsid

when using shepherd as user, programs like xorg server, 
potentially any
program which ends up interacting with user, require terminal to 
remain.

currently, setsid is called unconditionally from exec-command 
function,
making it impossible to have such process.

this adds keep-tty flag to make-forkexec-constructor up to 
exec-command
function where setsid is actually called.