Since gnulib commit 6954995d unistd.h is included via stdlib.h
on BSD systems, which requires config.h to be included first.

Add config.h to the files that use it.

Part of this commit reverts commit 6ee918deSigned-off-by: NJán Tomko <jtomko@redhat.com>
Reviewed-by: NErik Skultety <eskultet@redhat.com>