Add support for the more standard LIB_INSTALL_DIR variable

Pevents collisions with the original libgit, which also exports those
exact symbols.

Fixes #822

Not all delta bases are available on the first try. By delaying
resolving all deltas until the end, we avoid decompressing some of the
data twice or even more times, saving effort and time.

The correct way to advertise out capabilities is by appending them to
the first 'want' line, using SP as separator, instead of NUL as the
server does. Inconsistent documentation lead to the use of NUL in
libgit2.

Fix this so we can request much more efficient packs from the
remote which reduces the indexing time considerably.

More revparse <3

refs: fix a memory leak

    passing 0 to git_strol(32|64) let the implementation guess if it's
    dealing with an octal number or a decimal one.

    Let's make it safe and ensure that both 'HEAD@{010}' and 'HEAD@{10}'
    point at the same commit.

khash.h was globally #define'ing "inline" which messes with
other files.  Let's keep it as "kh_inline".

Some objects were added in another PR

odb: add git_odb_foreach()

Fix revwalk email parsing

Revparse refactoring: a start

Assorted goodies

refs and revparse love <3

This added a flag to the `git_repository_set_workdir()` function
that enables generation of a `.git` gitlink file that links the
new workdir to the parent repository.  Essentially, the flag tells
the function to write out the changes to disk to permanently set
the workdir of the repository to the new path.

If you pass this flag as true, then setting the workdir to something
other than the default workdir (i.e. the parent of the .git repo
directory), will create a plain file named ".git" with the standard
gitlink contents "gitdir: <repo-path>", and also update the
"core.worktree" and "core.bare" config values.

Setting the workdir to the default repo workdir will clear the
core.worktree flag (but still permanently set core.bare to false).

BTW, the libgit2 API does not currently provide a function for
clearing the workdir and converting a non-bare repo into a bare one.

Adding a new config iteration function that let's you iterate
over just the config entries that match a particular regular
expression.  The old foreach becomes a simple use of this with
an empty pattern.

This also fixes an apparent bug in the existing `git_config_foreach`
where returning a non-zero value from the iteration callback was
not correctly aborting the iteration and the returned value was
not being propogated back to the caller of foreach.

Added to tests to cover all these changes.