libc: group database

[b0inbot]

While porting nginx, i noticed that all the group db libc functions are missing.

Started an initial port which is just mirroring the getpwent work. https://git.sr.ht/~boin/genode/commit/5d31cd9d39879a94b3a830a178447be0bad62d4f .

[nfeske]

Please keep in mind that Genode has no notion of user and group IDs.

To make 3rd-party code happy, we could in principle extend the libc's configuration interface with a way to specify some (pseudo) values, similarly to how we handle the pid value in libc/internal/config.h.

[b0inbot]

Thanks! That's clear . I implemented this the same way the passwd is implemented, via <group> config, since the posix and BSD APIs are of group and passwd are mirrors.

#5801