wordexp: shell expansion support

The wordexp module implements word expansion using shell semantics, similar to
POSIX wordexp(3). Word expansion is performed with the platform-specific system
shell, which is generally compatible with POSIX sh(1) on Unix-like systems.

When used with a POSIX shell, the IFS variable is unconditionally unset in the
environment, causing the shell to assume the default value of " \t\n".

Note that, by design, this module runs arbitrary shell commands from
user-supplied inputs. It must only be used in a trusted environment.
