gitian: add libz-dev dependency package for linux boost

Boost iostreams was picking up libz-dev in VirtualBox, as the recommended
way to build is now to make a VM with all dependency packages installed.

This caused a divergence between KVM/LXC build and VirtualBox
build results.

Fix this in the simplest possible way: add the libz-dev package.
Don't use PRIx64 formatting derives from inttypes.h

As the tinyformat-based formatting system (introduced in b77dfdc) is
type-safe, no special format characters are needed to specify sizes.

Tinyformat can support (ignore) the C99 prefixes such as "ll" but
chokes on MSVC's inttypes.h defines prefixes such as "I64X". So don't
include inttypes.h and define our own for compatibility.

(an alternative would be to sweep the entire codebase using sed -i to
get rid of the size specifiers but this has less diff impact)
Make the signrawtransaction docs explicit that the arguments can be 'null'.
Merge pull request #3731 from gmaxwell/signrawnulldocs

Make the signrawtransaction docs explicit that the arguments can be 'null'
fix broken link to contrib/devtools
Merge pull request #3733

6fe6a44 fix broken link to contrib/devtools (freewil)
Merge pull request #3728

6c0276a gitian: add libz-dev dependency package for linux boost (Wladimir J. van der Laan)
Merge pull request #3729

d5f1e72 Don't use PRIx64 formatting derives from inttypes.h (Wladimir J. van der Laan)