[Ohrrpgce] SVN: teeemcee/11304 Split out findhero(-1) into first_used_slot_in_party() function

[subversion at HamsterRepublic.com]

teeemcee
2019-09-02 19:05:54 -0700 (Mon, 02 Sep 2019)
330
Split out findhero(-1) into first_used_slot_in_party() function

This has the effect of changing script commands like findhero, lockhero,
swapinhero, etc to no longer accept -1 as a hero id (taking the first
hero). Note that in Etheldreme and earlier (before r10439), you couldn't use -1
like that, instead -2 acted as "any hero".
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U   wip/game.bas
U   wip/game.bi
U   wip/moresubs.rbas