Use __strtod() on mingw

As strtod() is unable to parse NaN and Inf

Signed-off-by: Steve Bennett <steveb@workware.net.au>
Fix jim-exec.c compiler warnings on mingw

Signed-off-by: Steve Bennett <steveb@workware.net.au>
Improve file copy -force identical file check

On mingw we can't rely on dev+ino check

Signed-off-by: Steve Bennett <steveb@workware.net.au>