Daemon update

[Ludovic Courtès]

Commit 1303a4a fixes a use-after-free in the daemon that manifested when
compiling it with GCC 5.1 (string destructors were called before
‘execve’ calls that used the underlying ‘char *’ pointers.)

The fix is a backport of an upstream commit.  I plan to update our copy
of the Nix daemon code eventually, but it seemed wiser to do that after
the release, hence this backport.

Please report any issues.

Thanks,
Ludo’.