Add the exec builtin

Problem: The exec builtin does not exist

Solution: Implement the exec builtin as a simple wrapper around theexecve syscall.

Changes introduced by this pull request:

• Add the necessary functions to builtin
• Add the necessary interfaces for the execve syscall
  • Add sys::unix::execve
  • Add sys::redox::execve

TODOs:

• exec hits an assertion here when run from the original shell on redox. For example:

# Works
echo -e "#!/usr/bin/ion\nexec ion" > ./test.ion
ion ./test.ion
# Hits assertion
exec ion

Resolution: This occurred due to the context history not being prepped for exit.

Fixes:

• #608 (closed)