installer: added 'make upgrade' target

installer: added 'make upgrade' target

INTRANET_TMPL_DIR
INTRANET_WWW_DIR
OPAC_TMPL_DIR
OPAC_WWW_DIR
KOHA_CONF_DIR
OPAC_TMPL_DIR
OPAC_WWW_DIR
PAZPAR2_CONF_DIR
ZEBRA_CONF_DIR

For each directory on the list, if an existing file
installed file is different from the version
coming from the new package, it is copied to a backup file
whose name will have the suffix "_koha_<old_version_number>"
or "_upgrade_backup", depending on whether
the --prev-install-log option was used or not
during 'perl Makefile.PL'.

The directories whose files were backed up were
chosen because they contain configuration and
HTML files that a non-programmer Koha user
is likely to change.  Consequently, the backup
files produced by 'make upgrade' are not a substitute
for doing a complete backup of one's Koha
installation before upgrading.

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Joshua Ferraro <jmf@liblime.com>