Userland: useradd utility

This commit implements the `useradd` utility that is found on most,
if not all *NIX systems. It allows the root user to add new users
to the password file found in `/etc/passwd`, thereby making
it easier to manipulate the file.

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

@@ -0,0 +1,50 @@
+## Name
+
+useradd - add a new user to the system password file
+
+## Synopsis
+
+```**sh
+# useradd [options] <login>
+```
+
+## Description
+
+This program uses adds a new user to the system.
+
+By default, the user will be added to the **users** group (which has a GID of 100).
+
+This program must be run as root.
+
+## Options
+
+* `-u`, `--uid` _uid_: The user identifier for the new user. If not specified, an unused UID above `1000` will be auto-generated.
+* `-g`, `--gid` _gid_: The group identifier for the new user. If not specified, it will default to 100 (the **users** group).
+* `-s`, `--shell` _path-to-shell_: The shell binary for this login. The default is `/bin/Shell`. 
+* `-m`, `--create-home`: Create the specified home directory for this new user.
+* `-d`, `--home-dir` _path_: Set the home directory for this user to path. By default, this is `/home/username`, where `username` is the value of login.
+* `-n`, `--gecos` _general-info_: GECOS information about this login. See [Wikipedia](https://en.wikipedia.org/wiki/Gecos_field) for more information.
+
+## Exit Values
+
+* 0 - Success
+* 1 - Couldn't update the password file
+* 3 - Invalid argument to option
+* 4 - UID already in use
+* 12 - Couldn't create home directory
+
+## Files
+
+* `/etc/passwd` - new user information (such as UID and GID) is appended to this file.
+* `/home/` - user home directroy is created here if the `-m` flag is specified.
+
+## Examples
+
+```sh
+# useradd -u 300 -m kling
+# useradd -m -u 400 --gid 200 --gecos "Sergey Bugaev" bugaevc
+# useradd quaker
+# useradd --gid 1000 -d /tmp/somedir -n "Dan MacDonald" danboid
+# useradd --create-home supercomputer7
+```
+