perl: Include "guestfs-internal-frontend.h" in XS file.

Also undefine '_' symbol defined by Perl's internal headers.

This fixes commit ec3b75e5ffd1abea1400f6609ae9c61eab0dfa71.
ruby: Include "guestfs-internal-frontend.h" in extension.

Also undefine '_' symbol defined by Ruby's internal headers.

This fixes commit ec3b75e5ffd1abea1400f6609ae9c61eab0dfa71.
fish: Be stricter about boolean values.

Previously I noticed that bfan used this command without any error:

><fs> hivex-open /WINDOWS/system32/config/software write:ture
                                                   ^^^^^^^^^^

This was because the code allowed any string to be evaluated as a
boolean.

The new code is stricter.  It allows the following strings only case
insensitive (everything else is an error):

  1
  true
  t
  yes
  y
  on

  0
  false
  f
  no
  n
  off