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Our one supported compiler always defines the double-underscore variant, so always use that. Noticed by Bruce Evans after a report by John Capo.
Reduce _JBLEN for the i386 to what is actually used. Encapsulate jmp_buf and sigjmp_buf in structs. Enlarge jmp_buf to the same size as sigjmp_buf. Declare *longjmp as non-returning. Remove stale comments about sig*jmp not being implemented.