- execute.c: bpf functions were in the middle of an #if HAVE_SECCOMP
block for no reason
- test-fd-util.c: make seccomp-util.h includable without depending on
<seccomp.h>, and make is_seccomp_available() hardcoded to returning
false in this case.
Also fix a stray DEFINED() -- HAVE_SECCOMP is defined as 0, so normal
#if should be used like everywhere else.
*ret_culprit should be set if ret_culprit has been passed a non-null value,
checking the previous *ret_culprit value does not make sense.
This would cause the culprit to not properly be assigned, leading to
pid1 crash when a unit could not be stopped.
Fixes: #21476