SPDX headers

[Dirk Hohndel]

As some may have noticed, I'm in Europe this week, more precisely in
Barcelona at the FSFE Legal and Licensing Workshop. Fascinating
presentations... 30 minutes of a legal analysis of exhaustion of rights
and first sale doctrine. Truly riveting. But some other sessions were
somewhat more practical in nature, including one about per file license
information (co-lead by Greg K-H) and the role that SPDX can play in this.

While most of our files are under the GPLv2, some aren't. And almost none
of them actually state what license they are under. While following some
of the sessions today, I multitasked and tried to add SPDX headers to most
of our files. You can find the series of commits here:

https://github.com/Subsurface-divelog/subsurface/commits/spdx

While I think this is the right thing to do, I wonder if there's anyone
here who has a strong negative reaction (and would love if people could
double check that I didn't miss anything / didn't mislable anything).

Or, if no one cares, I can just merge it as is :-)

Thanks

/D