Thread safety for the refdb_fs

Even more lenient remote parsing

netops: remove duplicate include

Configuration iterators redux

Windows XP updates

In order to be loaded, a remote needs to be configured with at least a `url` or a `pushurl`.

ENOTFOUND will be returned when trying to git_remote_load() a remote with neither of these entries defined.

This loads SRWLock APIs at runtime and in their absence (i.e. on
Windows before Vista) falls back on a regular CRITICAL_SECTION
that will not permit concurrent readers.

refs: add git_reference_is_tag

push: small documentation fix

9e9aee67 added an include <netinet/in.h> to fix the build on FreeBSD.
Sometime since then the same header is included ifndef _WIN32, so
remove the duplicate include.

fix tests on FreeBSD

238b7614 introduced a test for posix behaviour, but on FreeBSD some
of the structs and constants used aren't defined in <arpa/inet.h>.
Include the appropriate headers to get the tests working again on
FreeBSD.

This converts an internal lock from a write lock to a read lock
where write isn't needed, and also clarifies some doc things about
where various locks are acquired and how various APIs are intended
to be used.

This makes libgit2 require Windows Vista or newer if it is going
to be compiled with the THREADSAFE option

This is the first use we have of pthread_rwlock_t in libgit2.
Hopefully it won't cause any serious portability problems.

Skip UTF-8 BOM in binary detection

This adds thread safety to the refdb_fs by using the new
git_sortedcache object and also by relaxing the handling of some
filesystem errors where the fs may be changed out from under us.

This also adds some new threading tests that hammer on the refdb.

The refdb_fs implementation calls realloc directly on a reference
object when it wants to rename it.  It is not a public object, so
this doesn't mess with the immutability of references, but it does
assume certain constraints on the reference representation.  This
commit wraps that assumption in an isolated API to isolate it.

This adds two other APIs that I need to the sortedcache type.

This adds a convenient new data type for caching the contents of
file in memory when each item in that file corresponds to a name
and you need to both be able to lookup items by name and iterate
over them in some sorted order.  The new data type has locks in
place to manage usage in a threaded environment.

If there were symbolic refs among the loose refs then the code
to create packed-refs would fail trying to parse the OID out of
them (where Git just skips trying to pack them).  This fixes it.

When a git_buf contains a UTF-8 BOM, the three bytes comprising
that BOM are treated as unprintable characters.  For a small git_buf,
the three BOM characters overwhelm the printable characters.  This
is problematic when trying to check out a small file as the CR/LF
filtering will not apply.