Userland: Introduce the pmemdump utility

This utility helps to dump the physical memory space from /dev/mem.
It supports both read(2) and mmap(2) on it so we could use mmap(2) for
fast dumping of the memory, or read(2) when we need to read unaligned
physical regions.

Base/usr/share/man/man1/pmemdump.md

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+## Name
+
+pmemdump - dump physical memory
+
+## Synopsis
+
+```**sh
+$ pmemdump [-r] <offset> <length>
+```
+
+## Description
+
+Dump a portion of the physical memory space.
+
+
+## Options
+
+* `-r`: Dump from /dev/mem with `read(2)` instead of doing `mmap(2)` on it.
+
+## Examples
+
+```sh
+$ pmemdump -r 983040 65536 
+$ pmemdump 983040 65536 
+```
+
+## Notes
+
+The pmemdump utility opens the `/dev/mem` file, and gets a mapping by doing `mmap(2)`
+on it.
+
+Using the `-r` flag might be useful sometimes, especially when reading from an unaligned
+reserved physical memory region when trying to `mmap(2)` `/dev/mem` on the specified
+offset fails.
+
+## See also
+
+* [`mem`(4)](../man4/mem.md)