Add Windows MSVC compilation target and CI

Restructure compat layer: #include_next headers moved to posix/ for
MinGW, new standalone headers in msvc/ for MSVC (unistd.h, utime.h,
getopt.h). Add getopt() implementation, chmod/unlink/chdir compat
functions, MSVC CRT initializer for UTF-8 console, _pgmptr fix.

cli/src/compat/include/msvc/getopt.h

@@ -0,0 +1,10 @@
+/* Minimal POSIX getopt for MSVC */
+#ifndef _COMPAT_GETOPT_H
+#define _COMPAT_GETOPT_H
+
+extern char *optarg;
+extern int optind, opterr, optopt;
+
+int getopt(int argc, char *const argv[], const char *optstring);
+
+#endif