chg: let procutil maintain its own pagerpid Previously, chg.c maintains the pagerpid. Let's move it to procutil.c. Note: chg.c still have a pagerpid to decide whether to call attachio or not. In the future, attachio may be moved from hgc_open to hgc_runcommand, and hgc_runcommand handles both pager and attachio so we don't need to run attachio twice. And chg.c will be free of pagerpid.