sys: improve error and crash handling

sys: improve error and crash handling

Renames Sys_Error to Sys_Abort (now using the Engine Abort message) for
clarity (as distinct from the non-fatal Host_Error).

Includes the engine version in the Crash and Abort SDL dialogs.

Linux: includes the glibc backtrace in the Engine Crash SDL dialog.
Fixes CTRL+C not working while the SDL dialog is open.

Fixes a double Sys_Error/Sys_Abort when memory corruption was detected.

Windows: omits colours when printing a Crash to stderr (not supported).

Makes signal handling (mostly) POSIX async-signal-safe.

Fixes a slight bug in PRVM_ShortStackTrace().

Signed-off-by: bones_was_here <bones_was_here@xonotic.au>