Add 'corebinutils/df/' from commit '04298aa63d4ae1546bed105cb8cb0245b3646ee5'

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+# df
+
+Standalone musl-libc-based Linux port of FreeBSD `df` for Project Tick BSD/Linux Distribution.
+
+## Build
+
+```sh
+gmake -f GNUmakefile
+gmake -f GNUmakefile CC=musl-gcc
+```
+
+## Test
+
+```sh
+gmake -f GNUmakefile test
+gmake -f GNUmakefile test CC=musl-gcc
+```
+
+## Notes
+
+- Port strategy is Linux-native mount and stat collection, not a FreeBSD ABI shim.
+- Mount enumeration uses `/proc/self/mountinfo`; capacity and inode counts use `statvfs(3)` on each selected mountpoint.
+- FreeBSD `getmntinfo(3)`, `sysctl(vfs.conflist)`, `getbsize(3)`, `humanize_number(3)`, and `libxo` are removed from the standalone port.
+- `-l` is implemented with Linux-oriented remote-filesystem detection based on mount type, `_netdev`, and remote-style sources such as `host:/path` or `//server/share`.
+- Linux has no equivalent of FreeBSD's cached `statfs` refresh control, so `-n` fails with an explicit error instead of silently degrading.
+- Linux also has no `MNT_IGNORE` mount flag equivalent; as a result `-a` is accepted but has no additional effect in this port.