Makefile.am: fix config_test sources

A few of the source files config_test epends on had been forgotten.
item.h: make weight a signed long

This avoids having to do overflow checks when reading from the
configuration file.
base_type: fix base type item parsing

A bug introduced in 3e12d13 "base_type: move base types to
configuration" prevented items from being associated with base types
because the wrong string was looked up in the item name table. As no
item was ever added, some other implementation bugs were also hidden.

Fix the bugs and complete the implementation so items can be associated
with base types.
mtrandom: fix generation of negative numbers

Previously, mtrandom_int64() generated positive and negative numbers
randomly. This commit changes this to always give positive numbers if
the range is positive and always negative numbers if the range is
negative. This makes it a lot more useful.

This also adds mtrandom_long().
base: add items

Add item support to bases. Players can query the item list by using the
"trade" command.
base: add stringtree for item names

Use a stringtree to keep track of all items associated with a base for
quick and easy lookup.
universe: add global base list with lock

Add a universe-global list of all bases with an accompanying lock.
base_update: update items in store at bases

Bases can produce and consume items. A daily amount per item per base is
set in the configuration files. This is accomplished by running a
separate thread with a configurable frequency that goes over all items
in all bases and updating the amount as necessary.
bases: add produces iron ore to mining colony

The mining colony now produces iron ore.
base: parse item requirements

An item in store at a base can require other items to be in stock before
it can be produced or consumed. This commit enables parsing this from
the configuration files.
base_update: use item requirements

Support requiring other items to be in stock before producing or
consuming an item at a base.
bases: rename Mining colony to Mining colony (iron ore)

In the far future, multiple mining colonies are envisioned.
bases: add steel mill

The steel mill produces steel from iron ore.
base: generate random number of bases

Generate a random number of bases on each planet supporting bases. The
random algorithm is the same as when generating planets with the maximum
number of bases per planet set to three. Previously, all planets had ten
bases.
bases: add some more base types

These base types are the ones listed as future ideas in the old data
files.
cargo: add and use for base and base_type

cargo replaces inventory as the basis of building inventories (i.e.
cargo holds) of ships and bases. Replace base_item and base_type_item by
this common cargo struct, thus simplifying the code.
player: add buy and sell commands

A player can buy and sell items when docked at a base. The items are put
(or taken from) the ships inventory.
player: add credits

Add money (credits) to the buy and sell process and give each player a
given set of starting credits in conn_fulfixinit().