build-sys: move fdset.[ch] src/basic → src/shared (#3580)

build-sys: move fdset.[ch] src/basic → src/shared (#3580)

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.