gnu: aegis, quilt: Use file-5.20.

Suggested by Mark H Weaver.

* gnu/packages/version-control.scm (aegis): Use FILE-5.20 instead of FILE.
* gnu/packages/patchutils.scm (quilt): Likewise.
gnu: file: Add itself as a native input for cross-compilation.

Fixes a regression introduced in c198872b.
Reported by John Darrington.

* gnu/packages/file.scm (file)[self-native-input?]: New field.
substitute-binary: Be slightly less verbose when downloading.

* guix/scripts/substitute-binary.scm (guix-substitute-binary)<"--substitute">:
  Be slightly less verbose.
artwork: Update snapshot.

* gnu/artwork.scm (%artwork-repository): Update.
doc: Improve acknowledgments.

* doc/guix.texi (Acknowledgments): Expound a bit.
* THANKS: Mention artwork; thank Felipe & Nikita.
syscalls: Update /etc/mtab, not /etc/fstab.

* guix/build/syscalls.scm (remove-from-mtab): Replace "fstab" with
  "mtab".
services: user-unmount: Change directory to "/" before unmounting.

* gnu/services/base.scm (user-unmount-service): Add 'chdir' call.