Mark _resumecontext as dead. ANSIfy all declarations for it.

Remove clause 3 and 4 from TNF licenses

Sync all resumecontext.c versions with the changes originally made
by Matthias Drochner to i386 and alpha:

-check for a NULL uc_link before calling setcontext(2); do a real exit(3)
instead with exi

Sync all resumecontext.c versions with the changes originally made
by Matthias Drochner to i386 and alpha:

-check for a NULL uc_link before calling setcontext(2); do a real exit(3)
instead with exit handlers and all that
-If setcontext(2) returns (which it shouldn't), do an _exit(-1) rather
than relying on a sensible return value to use as exit code. Makes
it less likely that an ucontext corruption goes unnoticed.

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Merge the nathanw_sa branch.