Some callers -- like our test suite and the test suites of our language
bindings -- want to isolate the home directory to avoid accidentally
including the executing user's actual home directory data.

Previously, we combined the notion of a home directory and global
configuration -- now that this is separated, we provide users the
ability to configure both.

Users can specify a config file to be included in the home directory
using `~/filename` syntax. Instead of looking in the global
configuration location (which may be overridden), use the user's
_actual_ home directory. This allows callers to change the global
configuration location separately from the home directory.

Users can specify an attribute file to be included in the home
directory using `~/filename` syntax. Instead of looking in the
global configuration location (which may be overridden), use the
user's _actual_ home directory. This allows callers to change the
global configuration location separately from the home directory.

I spent an hour banging my head against this, when it was because the
remote didn't trust my key.

Use `git_sysdir_find_homedir_file` to identify the path to the home
directory's `.ssh/known_hosts`; this takes Windows paths into account by
preferring `HOME`, then falling back to `HOMEPATH` and `USERPROFILE`
directories.

Provide a mechanism to look up the user's home directory, using the same
mechanism that we use for locating the global configuration path (a
fancy name for saying "the home directory").

SSH known hosts lookups now use this, instead of simply looking at the
HOME environment variable, to support Windows-style home directory
lookups in `HOME`, `HOMEPATH`, or `USERPROFILE`.

The windows sysdir functions are libgit2-specific and for git
compatibility only; remove them from the shared util directory and bring
them into the libgit2 source tree.

Add `OPENSSL_API_COMPAT=0x10100000L` since we use the now-deprecated
low-level hash functions.

The builtin hash uses the code verbatim from rfc6234, including
prototypes for functions that we don't use (like hmac). Remove all
unused prototypes to avoid collisions with things that an operating
system might provide (like hmac).