chg: move signal and pager handling to a separate file In the future hgclient will deal with pager directly inside runcommand, so related signal handling stuff needs to be decoupled from chg.c. The signal handling and pager logic are coupled because we need to forward SIGPIPE when pager exits. So they are moved together, otherwise a global variable (pagerpid) is inevitable. This patch moves related functions from chg.c to procutil.c, which was marked as copied to maintain annotate history. The move is done without code modification for easy review, therefore `#include "procutil.c"` was introduced temporarily.