This allows containers created by "guix environment -CN" or by
"guix system container -N" to talk to the host nscd.
* gnu/system/file-systems.scm (%network-file-mappings): Add
"/var/run/nscd".
* gnu/build/shepherd.scm (default-mounts)[nscd-socket]: Remove.
* gnu/system/linux-container.scm (container-script)[nscd-run-directory]
[nscd-mapping, nscd-os, nscd-specs]: Remove.
[script]: Filter out from SPECS bind-mounts where the device does not
exist.
* guix/scripts/environment.scm (launch-environment/container)
[optional-mapping->fs]: New procedure.
[mappings]: Remove %NETWORK-FILE-MAPPINGS.
[file-systems]: Add %NETWORK-FILE-MAPPINGS here, filtered through
'optional-mapping->fs'.
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