Userland: Add base64 tool

Base/usr/share/man/man1/base64.md

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+## Name
+
+base64 - encode and decode to base64
+
+## Synopsis
+
+```**sh
+$ base64 [-d|--decode] [file]
+```
+
+## Description
+
+`base64` encodes or decodes to base64 the data in file `file` or from stdin if
+file is not specified or file is `-`.
+
+## Options
+
+* `-d|--decode`: Decode data
+
+## Examples
+
+```sh
+# base64 encode the text 'foo'
+$ echo 'A' | base64
+# base64 encode the content of foo.txt
+$ base64 hi.txt
+# base64 decode the text 'Zm9v'
+$ echo 'Zm9v' | base64 -d
+# base64 decode the content of foo.txt
+$ base64 -d foo.txt
+```