We do want to notify users compiling our source code early on if they
try to use C flags which aren't supported. Add a new macro `AddCFlag`,
which results in a fatal error in case the flag is not supported, and
use it for our fuzzing flags.

Like all our other internal code, we want to force the use of C90 for
our fuzzers. Do so by setting the "C_STANDARD" property of our fuzzing
targets.

Our layout uses names like "examples" or "tests" which is why the "fuzz"
directory doesn't really fit in here. Rename the directory to be called
"fuzzers" instead. Furthermore, we rename the fuzzer "fuzz_packfile_raw"
to "packfile_raw_fuzzer", which is also in line with the already
existing fuzzer at google/oss-fuzz.

While at it, rename the "packfile_raw" fuzzer to instead just be called
"packfile" fuzzer.

This change adds support for building a fuzz target for exercising the
packfile parser, as well as documentation. It also runs the fuzzers in
Travis to avoid regressions.

ci: run VSTS builds on master and maint branches

Windows: default credentials / fallback credential handling

ci: add VSTS build badge to README

ci: set PKG_CONFIG_PATH for travis

Homebrew's formula for openssl is "keg-only", which means it does not
install it into /usr/local.  On macOS builds, we need to set
PKG_CONFIG_PATH to include it.

CI: Refactor and introduce VSTS builds

revwalk: remove tautologic condition for hiding a commit

Instead of trying to run coverity builds during the regular PR process,
run them during a regularly scheduled cron process.  These only need to
run nightly, so it makes sense to bring them out of the PR process.

The contition cannot be reached with `commit->uninteresting` being true:
either a `break` or a `continue` statement will be hit in this case.

winhttp: retry erroneously failing requests

The leaks process is not good about handling children.  Ensure that its
child is `nohup`ed so that the grandparent shell won't wait for it to
exit.

On Linux (where we run valgrind) allocate a smaller buffer, but still an
insanely large size.  This will cause malloc to fail but will not cause
valgrind to report a likely error with a negative-sized malloc.

Keep the original buffer size on non-Linux platforms: this is
well-tested on them and changing it may be problematic.  On macOS, for
example, using the new size causes `malloc` to print a warning to
stderr.

Add a configurable limit to the max pack size that will be indexed

Script to set up dependencies on a macOS build system.

Sets up a linux host to prepare for a build.

Refactor citest.sh to enable local testing by developers.

Add citest.ps1 PowerShell script to run the tests.