config: Make work to build units independently configurable

config: Make work to build units independently configurable

The work required for build and repairs is traditionally a function of
build materials: 20 + lcm + 2*hcm for ships, planes and land units,
and (lcm + 2*hcm + oil + rad)/5 for nukes.  Make it independently
configurable instead, via new ship-chr, plane-chr, land-chr, nuke-chr
selector bwork, backed by new struct mchrstr member m_bwork, struct
plchrstr member pl_bwork, struct lchrstr member l_bwork, struct
nchrstr member n_bwork.  Keep the required work exactly the same for
now.

Clients that compute work from materials need to be updated.  Easy,
since build work is now exposed in xdump.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>