Initially, the setting has been solely used to enable the use of
`fsync()` when creating objects. Since then, the use has been extended
to also cover references and index files. As the option is not yet part
of any release, we can still correct this by renaming the option to
something more sensible, indicating not only correlation to objects.

This commit renames the option to `GIT_OPT_ENABLE_FSYNC_GITDIR`. We also
move the variable from the object to repository source code.

Verifying hashsums of objects we are reading from the ODB may be costly
as we have to perform an additional hashsum calculation on the object.
Especially when reading large objects, the penalty can be as high as
35%, as can be seen when executing the equivalent of `git cat-file` with
and without verification enabled. To mitigate for this, we add a global
option for libgit2 which enables the developer to turn off the
verification, e.g. when he can be reasonably sure that the objects on
disk won't be corrupted.

This can prevent FILE_SHARED_VIOLATIONS when used in tools such as TortoiseGit TGitCache and FILE_SHARE_DELETE, because files can be opened w/o being locked any more.

Signed-off-by: Sven Strickroth <email@cs-ware.de>

Rename `GIT_OPT_ENABLE_SYNCHRONIZED_OBJECT_CREATION` ->
`GIT_OPT_ENABLE_SYNCHRONOUS_OBJECT_CREATION`.

Allow users to enable `SYNCHRONIZED_OBJECT_CREATION` with a setting.

This change provides an option in git_libgit2_opt_t which can be used in git_libgit2_opts to turn off/on ofs_delta capability in libGit2

Fixups requested in #3912.

Most importantly, clarify what it means for HTTPS and SSH to be supported.

We changed the defaults on strict object creation - it is enabled by
default.  Update the documentation to reflect that.