Add a way to limit deserialized string lengths

and use it for most strings being serialized.
Merge pull request #4655

216e9a4 Add a way to limit deserialized string lengths (Pieter Wuille)
Add travis CI builds.
Tidy up pkg.m4 into src/m4. Add minimal bitcoin.pc.in.
Set-up for multi compiler builds.
Add libminiupnpc. Add make check.