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.