Don't assume that if guestfs_case_sensitive_path returns NULL, that it
means the file does not exist.

The (previously undefined) behaviour of case_sensitive_path was that a
NULL return meant "either the file doesn't exist or some other error".
However in commit 973581780d8a006f336684fef6762801402d775d this was
changed so that if the last element of the path didn't exist, it was
assumed to be a new file and the (non-NULL) path of the new file is
returned.

This change breaks code (including in libguestfs) which tries to use
case_sensitive_path as a dual-purpose call to fix-up a path for
Windows and test if the file exists.  Such code should be rewritten so
that it explicitly tests for file existence after calling
case_sensitive_path.

I examined all the calls to case_sensitive_path in libguestfs and
modified them where necessary.

This also makes the test for license "code" less strict, so it should
only match POD, not Perl comments.

This fixes commit 2f97bf873b64384835f257f8916bf1ebb2af62b4.

The new API splits orderly close into a two-step process:

  if (guestfs_shutdown (g) == -1) {
    /* handle the error, eg. qemu error */
  }
  guestfs_close (g);

Note that the explicit shutdown step is only necessary in the case
where you have made changes to the disk image and want to handle write
errors.  Read the documentation for further information.

This change also:

 - deprecates guestfs_kill_subprocess

 - turns guestfs_kill_subprocess into the same as guestfs_shutdown

 - changes guestfish and other tools to call shutdown + close
   where necessary (not for read-only tools)

 - updates documentation

 - updates examples

NB: This requires hivex >= 1.3.2 before this syntax becomes true.

Turn:

 =item B<-a> | B<--all>

into:

 =item B<-a>

 =item B<--all>

This gives a more natural-looking manual page, as well as making it
easier to directly link to these sections.

This adds support for various Registry keys under HKEY_USERS (user
preferences).

(1) HKEY_USERS\<SID>
    where <SID> is a User SID.

For example:

  # virt-win-reg Windows 'HKEY_USERS\S-1-5-19'

lists out the LocalService user's registry.

(2) HKEY_USERS\<username>
    where <username> is a Windows local username (this is a
    libguestfs extension).

For example:

  # virt-win-reg Windows 'HKEY_USERS\rjones'

lists out the user preferences of user 'rjones'.

HKU can be used as an abbreviation for HKEY_USERS.  Merging is also
supported.