gnu: libarchive: Fix several security issues.

* gnu/packages/backup.scm (libarchive)[replacement]: New field.
(libarchive/fixed): New variable.
* gnu/packages/patches/libarchive-7zip-heap-overflow.patch,
gnu/packages/patches/libarchive-fix-symlink-check.patch,
gnu/packages/patches/libarchive-fix-filesystem-attacks.patch,
gnu/packages/patches/libarchive-safe_fprintf-buffer-overflow.patch: New files.
* gnu/local.mk (dist_patch_DATA): Add them.