Support survey dates before 1970

[Graham]

It would be useful if Survex could be made to register dates earlier than 1970. At least as far back as 1940 would be good for me.

[Olly Betts]

The simplest solution for now seems to be to make the time value in the 3d file signed and relax the < 1970 check to be < 1902. That comfortably covers the date ranges people seem to have survey data for.

That'll trivially work on platforms with a signed time_t (which seems to be all the ones we care about), but it seems Microsoft's C runtime doesn't handle negative time_t well:

​http://code.google.com/p/y2038/wiki/AmazingDiscoveries

So I guess we'll need to provide our own mktime() replacement there. It's not a complex function, and I found an implementation here with a suitable licence:

​http://lynx.isc.org/lynx2.8.5/lynx2-8-5/src/mktime.c

[Olly Betts]

Fixed in r3519.

I bypassed all the time_t range issues by storing the date as the number of days since Jan 1st 1900, so survey dates from 1900-2078 are now accepted, and .3d files are smaller too.

[Olly Betts]

Setting milestone to 1.1.15, since that's the release this fix will be in.