Per manpage here is requirement for setgroups():
  Since glibc 2.19:
    _DEFAULT_SOURCE
  Glibc 2.19 and earlier:
    _BSD_SOURCE

CentOS7 uses glibc-2.17 and thus requires _BSD_SOURCE. Without it we get
following compilation error:

src/luks/udisks2/clevis-luks-udisks2.c:327:13: error: implicit declaration of function 'setgroups' [-Werror=implicit-function-declaration]
             if (setgroups(1, &gid) != 0) {
             ^

This reverts commit ba8fab2.

It turns out that the new (CentOS8) dracut/systemd integration does not
work well with CentOS7 environment.

Tested: installed new clevis rpms at several hosts, regenerated dracut
image with `dracut --force`, reboot the host and observe it unlocked.

With current implementation sss-encrypt opens a pipe to the child pin,
then tries to read this pipe *only once*. This might return only partial
data or not data at all in case if the child pin response is slow.

Fix it by reading the pipe until it is closed, and only then process the
data.

Also increase the response buffer from 1024 bytes to 4096 bytes to
accound for larger subpin responses that can be larger than 1024 bytes.

Fixes #389