build: silence -Wsign-conversion in FD_SET()/FD_ISSET()

[vszakats]

Silence toolchain bugs causing warnings in FD_SET()/FD_ISSET() calls.

Seen with older Cygwin/MSYS2 builds. Likely fixed on 2020-08-03 by:
https://cygwin.com/git/?p=newlib-cygwin.git;a=commitdiff;h=5717262b8ecfed0f7fab63e2c09c78991e36f9dd

Also seen on Alpine MUSL:

multi.c:1201:9: error: conversion to 'long unsigned int' from 'curl_socket_t' {aka 'int'} may change the sign of the result [-Werror=sign-conversion]
 1201 |         FD_SET(ps.sockets[i], read_fd_set);
      |         ^~~~~~
multi.c:1201:9: error: conversion to 'long unsigned int' from 'curl_socket_t' {aka 'int'} may change the sign of the result [-Werror=sign-conversion]
multi.c:1203:9: error: conversion to 'long unsigned int' from 'curl_socket_t' {aka 'int'} may change the sign of the result [-Werror=sign-conversion]
 1203 |         FD_SET(ps.sockets[i], write_fd_set);
      |         ^~~~~~
multi.c:1203:9: error: conversion to 'long unsigned int' from 'curl_socket_t' {aka 'int'} may change the sign of the result [-Werror=sign-conversion]

Ref: https://github.com/curl/curl/actions/runs/8867959370/job/24347027402?pr=13489#step:31:373

And on OmniOS:

example/direct_tcpip.c:251:9: warning: conversion to 'long unsigned int' from 'libssh2_socket_t' {aka 'int'} may change the sign of the result [-Wsign-conversion]
    251 |         FD_SET(forwardsock, &fds);
        |         ^~~~~~
example/direct_tcpip.c:251:9: warning: conversion to 'long unsigned int' from 'libssh2_socket_t' {aka 'int'} may change the sign of the result [-Wsign-conversion]
example/direct_tcpip.c:251:9: warning: conversion to 'long unsigned int' from 'long int' may change the sign of the result [-Wsign-conversion]
example/direct_tcpip.c:251:9: warning: conversion to 'long int' from 'long unsigned int' may change the sign of the result [-Wsign-conversion]

Ref: https://github.com/libssh2/libssh2/actions/runs/8854199687/job/24316762831#step:3:2021

Ref: 5b9955e #13501
Ref: 95a882d #12557
Cherry-picked from #13489
Closes #13896

---

The fix is ugly. Perhaps creating a macro for it would be nicer, and then putting the original ones on the checksrc blocklist. [macro will not work, wrapper function problematic to match types, and performance.]

[vszakats]

Should this one remain or be deleted? It's a pattern already done in examples.

[bagder]

I think this is another sign-conversion warning that does not help us and where the fix makes the code harder to read.

[vszakats]

What about the pre-existing suppression in the example sendrecv.c, and one more in the open PR #12980? Should those be deleted?

[vszakats]

What about the pre-existing suppression in the example sendrecv.c, and one more in the open PR #12980? Should those be deleted?

Any guidance on that? Perhaps we want to keep doing this in examples because people with -Wconversion (which implies -Wsign-conversion) can bump into this warning?