New APIs: set-tmpdir, get-tmpdir, set-cachedir, get-cachedir.

The current code has evolved over time and has a number of problems:

(a) A single environment variable ($TMPDIR) controls the
location of several directories.

(b) It's hard for the library user to predict which directory
libguestfs will use, unless the user simulates the same internal steps
that libguestfs performs.

This commit fixes these issues.

(a) Now three environment variables control the location of all small
temporary files, and the appliance cache:

  For temporary files: $LIBGUESTFS_TMPDIR or $TMPDIR or /tmp.

  For the appliance cache: $LIBGUESTFS_CACHEDIR or $TMPDIR or /var/tmp.

The user can also set these directories explicitly through API calls
(guestfs_set_tmpdir and guestfs_set_cachedir).

(b) The user can also retrieve the actual directories that libguestfs
will use, by calling guestfs_get_tmpdir and guestfs_get_cachedir.
These functions are also used internally.

This commit also:

 - reworks the internal tmpdir code

 - removes the internal (undocumented) guestfs_tmpdir call (replacing
   it with calls to the documented guestfs_get_tmpdir API instead)

 - changes the ./run script to set LIBGUESTFS_TMPDIR and
   LIBGUESTFS_CACHEDIR

 - adds a test

 - fixes a few places like libguestfs-make-fixed-appliance which
   depended on $TMPDIR

This fixes commit 7786d56db8c22413949f98ef6b15fe0ea367d195.

New API: list-disk-labels

Allow the user to pass an optional disk label when adding a drive.

This is passed through to qemu / libvirt using the disk serial field,
and from there to the appliance which exposes it through udev,
creating a special alias of the device /dev/disk/guestfs/<label>.
Partitions are named /dev/disk/guestfs/<label><partnum>.

virtio-blk and virtio-scsi limit the serial field to 20 bytes.  We
further limit the name to maximum 20 ASCII characters in [a-zA-Z].

list-devices and list-partitions are not changed: these calls still
return raw block device names.  However a new call, list-disk-labels,
returns a hash table allowing callers to map between disk labels, and
block device and partition names.

This commit also includes a test.

When building supermin.d/daemon.img, use 'tmp-d' instead of 'tmp'
as the name of the temporary directory.

This is just code motion.

Transscribe many hivex(3) APIs into the libguestfs API.

There is one hive handle per libguestfs handle, as with Augeas.

Note that hivex uses iconv_open for some APIs (eg. hivex_value_string).
But since we delete all the i18n files from the appliance, this
doesn't work -- iconv_open returns EINVAL.  Therefore hivex APIs which
require iconv cannot be bound in the daemon.

This adds standard LICENSE and BUGS sections to all of the man pages
that are processed by podwrapper.

Modify all the calls to $(PODWRAPPER) to add the right --license
parameter according to the content.  Note that this relaxes the
license on some code example pages, making them effectively BSD-style
licensed.

section.

Ensure each man page contains consistent COPYRIGHT and AUTHOR
sections.

Remove the LICENSE section.  We will add that back in podwrapper in a
later commit.

There had been a virtual package "diff" that depended on diffutils, but that's gone in wheezy/sid, too.

This adds ~10M to the appliance (328M -> 338M).

However for virt-rescue these are frequently requested tools.

febootstrap >= 3.19 will no longer exclude the kernel package
by default.