Custom transport: minor cleanups

 * Move the transport registration mechanisms into a new header under
   'sys/' because this is advanced stuff.
 * Remove the 'priority' argument from the registration as it adds
   unnecessary complexity.  (Since transports cannot decline to operate,
   only the highest priority transport is ever executed.)  Users who
   require per-priority transports can implement that in their custom
   transport themselves.
 * Simplify registration further by taking a scheme (eg "http") instead
   of a prefix (eg "http://").

Don't include the unreadable tests on win32

Allow NULL error message prefix when class=GITERR_OS

config: a multiline var can start immediately

In the check for multiline, we traverse the backslashes from the end
backwards and int the end assert that we haven't gone past the beginning
of the line. We make sure of this in the loop condition, but we also
check in the return value.

However, for certain configurations, a line in a multiline variable
might be empty to aid formatting. In that case, 'end' == 'start', since
we ended up looking at the first char which made it a multiline.

There is no need for the (end > start) check in the return, since the
loop guarantees we won't go further back than the first char in the
line, and we do accept the first char to be the final backslash.

This fixes #2483.

Typo in timer constants

Don't report status on named pipes

Git skips entries in directories that are not S_ISDIR, S_ISREG, or
S_ISLNK, so let's make libgit2 do the same thing.

Compatibility/Portability cleanup

The effect of this would be that various update callbacks would
not be made at the correct interval.

* Removes mingw-compat.h
* Cleans up separation of compiler/platform idiosyncrasies
* Unifies mingw/msvc stat structures and functions
* (Tries to) hide more compiler specific implementation details (even in our internal API)

Solaris doesn't necessarily have stdint.h, use inttypes.h

Work around strict aliasing in array growth

This works around strict aliasing rules letting some versions of
GCC (particularly on RHEL 6) thinking that they can skip updating the
size of the array when calculating the next element's offset.

Preallocating two commits doesn't make much sense as leaving allocation
to the first array usage will allocate a sensible size with room for
growth.

This preallocation has also been hiding issues with strict aliasing in
the tests, as we have fairly simple histories and never trigger the
growth.

Don't allow conflicts by default

Fix git status list new unreadable folder

Rename git_cherry_pick to git_cherrypick

Export git_buf_text_is_binary and git_buf_text_contains_nul.

When adding new config section, handle config file not ending with LF

Handle local file:/// paths on Windows