[2.4] Fix missing rresvport function on systems using musl C library

[dgsga]

This commit:

• Fixes the missing rresvport function on linux systems using musl, credit @daimh

[dgsga]

@rdmark
The 2.4 branch now compiles fully on Alpine with both meson and autotools but we still have the issue of atalkd not loading correctly when built by meson. I also have no way of testing whether the new rresvport function works on Alpine. I found the code here, created for the very same reason:

https://github.com/daimh/sge/blob/master/source/libs/comm/glibc_rcmd.c

There is also a good overlap with the licensing :)

Any assistance with the atalkd issue is welcome, I have created the dgsga-alpine-atalkd branch to work on this issue

[rdmark]

@dgsga Great job! I will run some tests later today.

The MUSL detection logic is neat. Maybe in the long run when our local rresvport implementation has been "proven" we can use that across the board rather than the likely-to-be-deprecated glibc one.

[dgsga]

@dgsga Great job! I will run some tests later today.

The MUSL detection logic is neat. Maybe in the long run when our local rresvport implementation has been "proven" we can use that across the board rather than the likely-to-be-deprecated glibc one.

@rdmark, I have separated out the different issues so they each have a separate PR (for easier tracking). So this PR now only contains the rresvport fix

[sonarcloud]

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