Move etc/shells to lib/libc/gen/shells

[brd]

Summary

Switch to using CONFS to handle install.

[brd]

After discussion with Will, we decided libc was a better place for shells,
since that is where getusershell(3) comes from.

[will]

Summarizing discussion:

• chpass is the program that implements chsh which modifies /etc/shells, so it makes sense to install with this program.
• However, one could, strictly speaking, choose not to install chpass on a system, either because it will never update the shells list, or they edit it through other means.
• Additionally, /etc/shells is read by getusershell(3) in libc, which _is_ required by some programs, in particular su(1). So it could make more sense to install this file with libc instead.