Userland: `sed` not POSIX compatible

[cofinalsubnets]

sed replace is a wrapper around LibRegex replace which doesn't recognize & in replacements. There may be other differences from Posix, I haven't checked exhaustively. In the interest of offering a fix i'd like to request opinions

• should LibRegex replace act like sed by default?
• if not, should this behavior be covered under an existing regex posix option flag or a new one?

[nico]

@alimpfard