Handling of BANPIPE_MODULE_PATH on windows

[cth]

Semicolon is used as separator under windows - need reliable way to detect if OS is windows

[pmoura]

Logtalk checks if the environment variable COMSPEC is defined. Apparently, all Windows versions define this variable but it's not expected on POSIX systems. Other environment variables usually only defined on Windows are HOMEDRIVE and HOMEPATH.