chg: add procutil.h This patch adds a formal header procutil.h for procutil.c, and changes Makefile to build procutil.c independently.

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.

chg: decouple hgclient from setuppager procutil should not depend on hgclient. This patch makes the pager handling part independent from hgclient.

chg: decouple hgclient from setupsignalhandler procutil should not depend on hgclient. This patch makes the signal handling part independent from hgclient.

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.

keepalive: rewrite readline() The old method was performing string concatenation, which is slower than collecting raw chunks in a list and joining at the end.

keepalive: remove limit argument from readline() It is unused and adds complexity.