Windows has its own ftruncate() called _chsize_s().

p_mkstemp() is changed to use p_open() so we can make sure we open for
writing; the addition of exclusive create is a good thing to do
regardless, as we want a temporary path for ourselves.

Lastly, MSVC doesn't quite know how to add two numbers if one of them is a
void pointer, so let's alias it to unsigned char.C

Some OSs cannot keep their ideas about file content straight when mixing
standard IO with file mapping. As we use mmap for reading from the
packfile, let's make writing to the pack file use mmap.

Win32: Supply _O_NOINHERIT when calling _wopen

Remote deletion

When we delete a remote, we also need to go through its fetch refspecs
and remove the references they create locally.

pack: init the cache on packfile alloc

Better global search path sandboxing in tests

When running multithreaded, it is not enough to check for the offmap
allocation. Move the call to cache_init() to packfile allocation so we
can be sure it is always allocated free of races.

This fixes #2355.

There are a number of tests that modify the global or system
search paths during the tests.  This adds a helper function to
make it easier to restore those paths and makes sure that they
are getting restored in a manner that preserves test isolation.