chg: extract the logic of setting FD_CLOEXEC to a utility function Setting FD_CLOEXEC is useful for other fds such like lockfd and sockdirfd, move the logic from hgc_open to util.

chg: add fchdirx as a utility function As part of the series to support long socket paths, we need to use fchdir and check its result in several places. Make it a utility function.

chg: check lockfd at freecmdserveropts We check for sockdirfd at freecmdserveropts but not lockfd, which is a bit strange to people new to the code. Add a comment and an assert to make it clear that lockfd should be closed earlier.

chg: add sockdirfd to cmdserveropts As part of the series to support long socket paths, we need to add the fd of the directory to the cmdserveropts structure so we can use basenames instead of full paths for sockname, redirectsockname, and lockfile.