krb5: fix socket confusion, MSVC compiler warnings

[vszakats]

• fix socket/sockindex confusion on writes:

  The callstack used to end with Curl_write_plain() accepting a socket
  till 7.87.0. This call got swapped for Curl_conn_send(), expecting
  a sockindex. socket_write() was updated accordingly. Its callers
  missed it and continued operating on sockets: do_sec_send(),
  sec_write(), passing it down the stack and Curl_conn_send()
  resolving it as if it was a sockindex.
  It affected FTP Kerberos authentication.

  Discovered through MSVC warnings:

  curl\lib\krb5.c(652,28): warning C4244: 'function': conversion from 'curl_socket_t' to 'int', possible loss of data
  curl\lib\krb5.c(654,28): warning C4244: 'function': conversion from 'curl_socket_t' to 'int', possible loss of data
  curl\lib\krb5.c(656,26): warning C4244: 'function': conversion from 'curl_socket_t' to 'int', possible loss of data
  curl\lib\krb5.c(657,26): warning C4244: 'function': conversion from 'curl_socket_t' to 'int', possible loss of data
  curl\lib\krb5.c(665,24): warning C4244: 'function': conversion from 'curl_socket_t' to 'int', possible loss of data
  curl\lib\krb5.c(666,24): warning C4244: 'function': conversion from 'curl_socket_t' to 'int', possible loss of data 
  

  Ref: https://github.com/curl/curl/actions/runs/11846599621/job/33014592805#step:9:32

  Follow-up to 5651a36 Conn_recv/send_plain refactoring, USE_RECV_BEFORE_SEND_WORKAROUND in cf-socket #10280
  Bug: GHA/windows: add gssapi to openssl job #15549 (comment)
  Fixes int/socket confusion warnings #15582

• fix uninitialized buffer:

  curl\lib\krb5.c(288,1): warning C4701: potentially uninitialized local variable '_gssresp' used
  

  Ref: https://github.com/curl/curl/actions/runs/11848626645/job/33020501026?pr=15585#step:9:31

• silence unreachable code compiler warning:

  curl\lib\krb5.c(370,1): warning C4702: unreachable code
  

  Ref: https://github.com/curl/curl/actions/runs/11848626645/job/33020501026?pr=15585#step:9:30

---

• drop GHA update in favor of GHA/windows: add gssapi to openssl job #15549
• split off other fixes to separate PRs? → 8115e79

[talregev]

Thank you for your fix! 🎉
I drop my PR in favor this PR.

[testclutch]

Analysis of PR #15585 at 7a430ccc:

Test 3028 failed, which has NOT been flaky recently, so there could be a real issue in this PR.

Generated by Testclutch