Preliminary DragonFly BSD support

[tuxillo]

Changes are untested in linux/darwin

[gdiazlo]

If jemalloc is the problem then, I would say the PR should address it as "enable / disable the use of jemalloc" instead of a "dragonfly bsd thing".

I don't like to personalize the build process for each OS, but instead, generalize it so it will work the same (yes I know we have already a Darwin thing, but it should disappear too eventually).

Can you change it to enable / disable jemalloc for all the builds and delete the specifics of dargonflybsd?

[tuxillo]

Not sure how you can turn on/off jemalloc when you have to specify LDFLAGS for each platform in flags.go.

[gdiazlo]

The code is already there, it differenciates between linux and osx. With a new flag which, instead of being called dbsd should be called nojemmalloc or simmilar. On the other hand, does jemalloc support landed on dbsd?