cygwin.c: fix several silly/terrible C errors

[mauke]

Errors fixed:

Not detected by the compiler, but the preprocessor version checks only looked at CYGWIN_VERSION_API_MINOR, ignoring
CYGWIN_VERSION_API_MAJOR.

cygwin.c: In function 'utf8_to_wide_extra_len':
cygwin.c:187:18: warning: initialization of 'size_t' {aka 'long unsigned int'} from 'size_t *' {aka 'long unsigned int *'} makes integer from pointer without a cast [-Wint-conversion]
  187 |     Size_t len = strlen(buf) + extra_len + 1;
      |                  ^~~~~~

extra_len was declared as a pointer, but used as an integer. I don't see how this could have ever worked.

cygwin.c: In function 'S_convert_path_common':
cygwin.c:339:38: warning: 'realloc' called on unallocated object 'wconverted' [-Wfree-nonheap-object]
  339 |             wconverted = (wchar_t *) realloc(&wconverted, newlen);
      |                                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
cygwin.c:357:39: warning: 'realloc' called on unallocated object 'converted_path' [-Wfree-nonheap-object]
  357 |             converted_path = (char *) realloc(&converted_path, newlen);
      |                                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Again, I don't see how this could have ever worked. We're passing the address of a local (stack) variable to realloc (instead of the value of the variable).

cygwin.c: In function 'wide_to_utf8':
cygwin.c:173:32: warning: pointer targets in passing argument 3 of 'Perl_utf16_to_utf8_base' differ in signedness [-Wpointer-sign]
  173 |     utf16_to_utf8((U8 *) wsrc, buf, wlen, &blen);
      |                                ^~~
      |                                |
      |                                char *

The argument is declared as U8 *, so this is mostly harmless, but a cast silences the warning anyway.

cygwin.c: In function 'utf8_to_wide_extra_len':
cygwin.c:194:19: warning: passing argument 2 of 'Perl_utf8_to_utf16_base' discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
  194 |     utf8_to_utf16(buf, (U8 *) wsrc, len, &wlen);
      |                   ^~~

This is arguably a bug in utf8_to_utf16, which doesn't declare its input argument as pointer-to-const. But it's also a char * vs U8 * type mismatch, so a cast silences both issues.

cygwin.c: In function 'S_convert_path_common':
cygwin.c:292:22: warning: initialization discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
  292 |         char *name = (direction == to_posix)
      |                      ^

This is a missing const in the declaration of a variable that points to string literals.

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• This set of changes does not require a perldelta entry.