lockfile: Linux: Fixed bug where need to change primary group of the dotlockfile process to the group of the dir to contain the lockfile, because dotlockfile otherwise reports a "permission denied" error (even though the directory is actually writable, dotlockfile thinks it isn't). Running dotlockfile with a different primary group is complicated because newgrp, the command that does this, does not pass arguments to the new process, so they must instead be passed via environment variables and a recursive invocation of lockfile (with the $inner recursion flag set). Additionally, exec cannot be used to propagate the PPID (needed by dotlockfile) because newgrp creates a new process rather than using exec, so it must be manually entered into the lockfile after dotlockfile runs.