don't hardcode /usr/share paths

don't hardcode /usr/share paths

Add a defines.mk file defining the standard default layout
for installation (PREFIX, DATAROOTDIR, LIBDIR etc.).

Add a $(JSDIR) variable and use env!("JSDIR") in place of
the hardcoded /usr/share/javascript/proxmox-backup path.

Have defines.mk include an *optional* local.mak to override
JSDIR to www/ during local development.

Add `make install` targets and remove debian/install.

Note that service files are now in $libdir/proxmox-backup/
as there's usually no point in starting them from the
command line, so /usr/sbin makes no sense.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>