Base: Add more manpages for command-line utilities

Base/usr/share/man/man8/dmesg.md

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+## Name
+
+dmesg - display the kernel log
+
+## Synopsis
+
+```**sh
+# dmesg
+```
+
+## Description
+
+`dmesg` displays the kernel log (as seen on serial debug output). Only messages printed by kernel processes are displayed.
+
+## Examples
+
+```sh
+$ dmesg
+```

Base/usr/share/man/man8/hostname.md

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+## Name
+
+hostname - print or set hostname
+
+## Synopsis
+
+```**sh
+$ hostname [hostname]
+```
+
+## Description
+
+`hostname` prints current host name. If the `hostname` argument is give and the user is a superuser, the utility sets the current host name to its value. The maximum hostname length is 64 characters. The hostname is not persisted after reboot.
+
+## Examples
+
+```sh
+# Print current hostname
+$ hostname
+courage
+
+# Set the new hostname
+# hostname foo
+
+# Print new hostname
+# hostname
+foo
+```

Base/usr/share/man/man8/umount.md

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+## Name
+
+umount - unmount file system
+
+## Synopsis
+
+```**sh
+$ umount <mountpoint>
+```
+
+## Arguments
+* `mountpoint`: File system path to unmount
+
+## Description
+
+`umount` run as root unmounts a file system mounted at specified `mountpoint`.
+
+## Examples
+
+```sh
+# umount /
+```
+
+## See also
+
+* [`mount`(8)](../man8/mount.md)