Task tracked at:
teams/frameworks-devs/kf6-workboard#2

It's no longer needed after [1], which ported away from
KToolInvocation::kdeinitExecWait.

KF5Config was implicitly pulled in by KF5Service and is actually
used.

[1] 35251057

See:
https://kde.org/info/security/advisory-20220131-1.txt
https://mail.kde.org/pipermail/kde-devel/2022-January/000943.html

The close_range function was added in glibc in version 2.34; for older
versions use syscall(SYS_close_range.....).

Also since close_range was first avaiable in Kernel 5.9, for older kernels
fallback to iterating over /proc/self/fd/ and closing the fd's manually
(using opendir/closedir/readdir as suggested by Fabian Vogt).

BUG: 449114

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Using waitForFinished() to block until kdesud is started; this matches the
behaviour of KToolInvocation::kdeinitExecWait().

After the previous commit, file descriptors of kdesud are closed on
exec().

kdesud calls fork() then terminates the calling/parent process (by calling
_exit(0) when the pid returned by fork() != 0).

- Set the socket fd to 3, and then explicitly close all file descriptors higher
  than 3 right before calling fork().

- Set O_CLOEXEC flag for the fd's used by pipe()

- Set SOCK_CLOEXEC flag for the socket

Thanks to Fabian Vogt for helping me / explainging through all that. :)

Instead of /usr/include/KF5/kdesu_version.h.

For more details see:
kservice!79

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More range-for.

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clang-tidy checks:
readability-isolate-declaration and readability-braces-around-statements

KF task: https://phabricator.kde.org/T14729

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