Compilation support on various BSDs

[kikadf]

I am a satisfied user of the MEGA service, thank you for your work. I use the mega-sdk (megacmd) on Linux, on FreeBSD, as well as with minor modifications DragonFly BSD, NetBSD, and OpenBSD. In the case of FreeBSD and NetBSD, I maintain the megacmd package that is available in their package repositories, but I use it with full functionality and without issues on all BSD systems.

I am sending some pull requests that make successful compilation on the various BSDs.

I understand that DragonFly, NetBSD, and OpenBSD are not officially supported platforms and there are no plans for that, however, only simple changes (that do not affect other platforms) are needed to make the mega-sdk compile on these systems as well.

I hope you find these acceptable and worth to merging.