Userland: Add an implementation of 'expr'

This implements all expressions except 'match', which errors out when executed.
Closes #1124?

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

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+## Name
+
+expr - evaluate expressions
+
+## Synopsis
+
+```**sh
+$ expr <expression>
+$ expr [--help]
+```
+
+## Description
+expr evaluates and prints the result of an expression as described below to standard output.
+
+An _expression_ may be any of the following:
+- `expr1 | expr2`
+    `expr2` if `expr1` is falsy, `expr1` otherwise.
+- `expr1 & expr2`
+    `expr1` if neither expression is falsy, `0` otherwise.
+- `expr1 < expr2`
+    `1` if `expr1` is less than `expr2`, `0` otherwise.
+- `expr1 <= expr2`
+    `1` if `expr1` is less than or equal to `expr2`, `0` otherwise.
+- `expr1 = expr2`
+    `1` if `expr1` is equal to `expr2`, `0` otherwise.
+- `expr1 = expr2`
+    `1` if `expr1` is not equal to `expr2`, `0` otherwise.
+- `expr1 => expr2`
+    `1` if `expr1` is greater than or equal to `expr2`, `0` otherwise.
+- `expr1 > expr2`
+    `1` if `expr1` is greater than `expr2`, `0` otherwise.
+- `expr1 + expr2`
+    arithmetic integral sum of `expr1` and `expr2`.
+- `expr1 - expr2`
+    arithmetic integral difference of `expr1` and `expr2`.
+- `expr1 * expr2`
+    arithmetic integral product of `expr1` and `expr2`.
+- `expr1 / expr2`
+    arithmetic integral quotient of `expr1` divided by `expr2`.
+- `expr1 % expr2`
+    arithmetic integral quotient of `expr1` divided by `expr2`.
+- `expr1 : expr2`
+    pattern match of `expr2` as a regular expression in `expr1` - currently not implemented.
+- `match expr1 expr2`
+    same as `expr1 : expr2`.
+- `substr expr1 expr2 expr3`
+    substring with length `expr3` of `expr1`, starting at `expr2`, indices starting at 1.
+- `index expr1 expr2`
+    index of `expr2` in `expr1`, starting at 1. 0 if not found.
+- `length expr1`
+    length of the string `expr1`
+- `+ token`
+    interpret `token` as a string, regardless of whether it is a keyword or an operator.
+- `( expr )`
+    value of `expr`
+
+Note that many operators will need to be escaped or quoted if used from within a shell.
+"falsy" means either the number 0, or the empty string.
+
+## Options
+
+* `--help`: Prints usage informations and exits.
+
+## Examples
+
+```sh
+$ expr 1 + 2 * 3          # = 7
+$ expr \( 1 + 2 \) = 3    # = 1
+$ expr substr foobar 1 3  # foo
+```
+
+## See also
+