Add DESTDIR support to Makefile

During packaging for distros most of them install under $DESTDIR instead
of the root directory. See:

http://www.gnu.org/prep/standards/html_node/DESTDIR.html
http://www.debian.org/doc/manuals/maint-guide/modify.en.html#destdir
http://www.freebsd.org/doc/en/books/porters-handbook/porting-prefix.html
Don't ignore CPPFLAGS on compilation

CPPFLAGS is part of the built-in rules for compilation. Although it is not
used by ipv6-toolkit itself, distros may pass flags on CPPFLAGS too to
comply with some policies.

Being part of the built-in rules, custom rules like used in the Makefile
should explicitly include CPPFLAGS. See:

http://en.wikipedia.org/wiki/CFLAGS
http://stackoverflow.com/questions/2754966/cflags-vs-cppflags

In the case of Debian, debuild passes flags for executable hardening.

http://lintian.debian.org/tags/hardening-no-fortify-functions.html
Fix of some spelling mistakes detected by Debian lintian
Fix manpage warnings reported by Debian lintian

Lintian threw 'cannot adjust' and 'cannot break' warnings. This patch
fixes them. See:

http://lintian.debian.org/tags/manpage-has-errors-from-man.html
Replaced most hyphens for minus signs in man pages

In a man page, hyphen and minus are different signs and can be rendered
differently depending on the output. This can cause examples to fail
for users that copy and paste examples from rendered man pages.

Hyphen is coded like this: -
Minus is coded like this: \-
Options like --help use minus, like this: \-\-help.

This was detected by Debian lintian. See:

http://lintian.debian.org/tags/hyphen-used-as-minus-sign.html