Linux Crashlog Support

[NicksWorld]

This PR adds functionality to DFHack that enables it to produce crashlogs (on Linux only) upon the majority of crashes (specifically ignoring those that occur post-shutdown).

There are a few topics I wish to address before considering it ready:

• BT_ENTRY_MAX can be any arbitrary value, is 25 lines a good limit, or should it be higher?
• Should this feature be togglable with an environment variable?
• A review of concurrency, as the implementation makes use of quite a few atomics and filedescriptors in an attempt to maintain async-signal safety in the signal handler. It passes the trials I've put it through, but there may be more edge cases.

As the implementation stands, I think the only possible hang would be if file io stalled whilst writing the crashlog. The crashlogs include a header that aims to make it clear these logs are produced by dfhack and not the vanilla game, which should hopefully prevent confusion.

An example generated crashlog is as follows:

Dwarf Fortress Linux has crashed!
Dwarf Fortress Version 53.03
DFHack Version 53.03-r1

Signal SIGSEGV
0> hack/libdfhack-debug.so(+0xc669da) [0x7fffe38669da]
1> /usr/lib/libc.so.6(+0x3e540) [0x7ffff6e3e540]
2> hack/libdfhack-debug.so(_ZN6DFHack16pointer_identity8lua_readEP9lua_StateiPvPKNS_13type_identityE+0x1b) [0x7fffe34301c1]
3> hack/libdfhack-debug.so(+0x83127c) [0x7fffe343127c]
4> hack/libdfhack-debug.so(+0x831749) [0x7fffe3431749]
5> hack/liblua53.so(+0x18769) [0x7fffe3f4c769]
6> hack/liblua53.so(+0x18afb) [0x7fffe3f4cafb]
7> hack/liblua53.so(+0x18b6e) [0x7fffe3f4cb6e]
8> hack/liblua53.so(+0x33b20) [0x7fffe3f67b20]
9> hack/liblua53.so(+0x35ceb) [0x7fffe3f69ceb]
10> hack/liblua53.so(_Z12lua_gettableP9lua_Statei+0xe3) [0x7fffe3f3ea8e]
11> hack/libdfhack-debug.so(+0x831bee) [0x7fffe3431bee]
12> hack/liblua53.so(+0x18769) [0x7fffe3f4c769]
13> hack/liblua53.so(+0x18afb) [0x7fffe3f4cafb]
14> hack/liblua53.so(+0x3b9e3) [0x7fffe3f6f9e3]
15> hack/liblua53.so(+0x18b10) [0x7fffe3f4cb10]
16> hack/liblua53.so(+0x18b6e) [0x7fffe3f4cb6e]
17> hack/liblua53.so(_Z9lua_callkP9lua_StateiilPFiS0_ilE+0xb5) [0x7fffe3f3fd62]
18> hack/libdfhack-debug.so(_ZN6DFHack21dfhack_lua_viewscreen8do_inputEP9lua_State+0x139) [0x7fffe38348d7]
19> hack/liblua53.so(+0x18769) [0x7fffe3f4c769]
20> hack/liblua53.so(+0x18afb) [0x7fffe3f4cafb]
21> hack/liblua53.so(+0x18b6e) [0x7fffe3f4cb6e]
22> hack/liblua53.so(+0xbde0) [0x7fffe3f3fde0]
23> hack/liblua53.so(+0x17a81) [0x7fffe3f4ba81]
24> hack/liblua53.so(+0x1942e) [0x7fffe3f4d42e]