Documentation: allow installing man pages to a user defined directory

Documentation/DocBook/Makefile hard codes the prefixed path to which you
can install the built man pages (/usr/local prefix). That's unfortunate
since the user may want to install to another prefix or location (for
example, a distribution packaging the man pages may want to install to a
random temporary location in the build process).

Be flexible and allow the prefixed path to which we install man pages to be
changed with the INSTALL_MAN_PATH environment variable (and use the same
default as other similar variables like INSTALL_HDR_PATH).

Signed-off-by: Herton R. Krzesinski <herton@redhat.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>