negotiate_sspi_auth: Fix crash in FormatMessage() (#2510)

[squidadm]

When FormatMessage() fails, negotiate_sspi_auth reports the failure
with

SEND2("NA * Windows error: %s", GetLastError());

GetLastError() returns a DWORD, so %s makes printf() -- and debug(),
through SEND2 -- treat the error number as a pointer and read from
it. The helper crashes on this path instead of answering Squid with
NA.

By then GetLastError() no longer holds the error being reported
either: FormatMessage() has replaced it with its own failure code,
ERROR_MR_MID_NOT_FOUND (317). Save the error before calling
FormatMessage(), and report that saved code when Windows has no
message text for it. When text is available, the existing path
still sends it.

Print the number with %lu and cast it to unsigned long. DWORD is
unsigned int on Cygwin, where the SSPI helpers are built, and
unsigned long on MinGW, so no single conversion without a cast fits
both. src/windows_service.cc prints GetLastError() the same way, to
keep the two in sync.

Found building 7.7 on Cygwin, where GCC 14 warns: format '%s' expects
argument of type 'char*', but argument 2 has type 'DWORD'. With this
change the file compiles with no format warnings under -Wall -Wextra
-Wformat=2 on Cygwin, and windows_service.cc's line compiles without
format warnings on MinGW GCC 16 even with -Wformat-signedness.