df: split POSIX, FreeBSD, NetBSD, OpenBSD, osx and GNU (#11870)

Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>

pages/openbsd/df.md

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+# df
+
+> Display an overview of the filesystem disk space usage.
+> More information: <https://man.openbsd.org/df.1>.
+
+- Display all filesystems and their disk usage using 512-byte units:
+
+`df`
+
+- Display all filesystems and their disk usage in human-readable form (based on powers of 1024):
+
+`df -h`
+
+- Display the filesystem and its disk usage containing the given file or directory:
+
+`df {{path/to/file_or_directory}}`
+
+- Include statistics on the number of free and used [i]nodes:
+
+`df -i`
+
+- Use 1024-byte units when writing space figures:
+
+`df -k`
+
+- Display information in a portable way:
+
+`df -P`