rust-chg: install signal handlers to forward signals to server I use sync::Once as a synchronization primitive because it's quite easy to use, and is good enough to prevent data race in these C functions.

rust-chg: remove SIGCHLD handler which won't work in oxidized chg Since pager is managed by the Rust part, the C code doesn't know the pager pid. I could make the Rust part teach the pid to C, but still installing SIGCHLD handler seems horrible idea since we no longer use handcrafted low-level process management functions. Instead, I'm thinking of adding async handler to send SIGPIPE at the exit of the pager.

rust-chg: extract signal handlers from chg/procutil.c abortmsgerrno() and debugmsg() are removed, and the public interface instead returns success/error status. Since signal handlers can't propagate errors, the result of kill() is just ignored.

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