MariaDB client - "system" command does not work on Windows

The reason for this is because USE_POPEN was disabled on Windows, a long long time ago, on whatever undocumented reasons -perhaps it did not work on Win95, when Monty tried it, or maybe it was fork+exec back then, we'll never know.

popen() works ok on Windows, apparently.

This "works on Unix only" forces users to do unnecessary workarounds, e.g when creating NTFS snapshot backups , see https://serverfault.com/a/360326/64221 . The only thing that is needed for easy backup'ing would be to insert a "vssadmin" command between FTWRL and UNLOCK TABLES.