build-sys: move fdset.[ch] src/basic → src/shared

[poettering]

It makes use of the sd_listen_fds() call, and as such should live in
src/shared, as the distinction between src/basic and src/shared is that the
latter may use libsystemd APIs, the former does not.

Note that btrfs-util.[ch] and log.[ch] also include header files from
libsystemd, but they only need definitions, they do not invoke any function
from it. Hence they may stay in src/basic.

[evverx]

The failure seems unrelated:

Cloning into 'systemd'...
warning: You appear to have cloned an empty repository.
sed: can't read debian/changelog: No such file or directory
blame: https://anonscm.debian.org/git/pkg-systemd/systemd.git
badpkg: rules extract failed with exit code 2

I'm merging.
(I think we should add this note:

    build-sys: split internal basic/ library from shared/

    basic/      can be used by everything
                cannot use anything outside of basic/

    libsystemd/ can use basic/
                cannot use shared/

    shared/     can use libsystemd/

somewhere

[poettering]

@evverx added a comment about the split now to CODING_STYLE in #3802. Please have a look.