Pack file verification

C++ style comment ("//") are not specified by the ISO C90 standard and
thus do not conform to it. While libgit2 aims to conform to C90, we did
not enforce it until now, which is why quite a lot of these
non-conforming comments have snuck into our codebase. Do a tree-wide
conversion of all C++ style comments to the supported C style comments
to allow us enforcing strict C90 compliance in a later commit.

We strive to keep an options structure to many functions to be able to
extend options in the future without breaking the API. `git_indexer_new`
doesn't have one right now, but we want to be able to add an option
for enabling strict packfile verification.

Add a new `git_indexer_options` structure and adjust callers to use
that.

The getline(3) function call is not part of ISO C and, most importantly,
it is not implemented on Microsoft Windows platforms. As our networking
example code makes use of getline, this breaks builds on MSVC and MinGW.
As this code wasn't built prior to the previous commit, this was never
noticed.

Fix the error by instead implementing a `readline` function, which
simply reads the password from stdin until it reads a newline
character.

The credentials callback reads the username and password via scanf into
fixed-length arrays. While these are simply examples and as such not as
interesting, the unchecked return value of scanf causes GCC to emit
warnings. So while we're busy to shut up GCC, we also fix the possible
overflow of scanf by using getline instead.

Format of a length of string to the correct format is:%.*s