Simplify - it's enough to set nctx.uc_link to &octx to restore old
context automatically when swapfunc() returns.

Add missing $ in the RCSID.

threadfunc - we do return from swapcontext() (since revision 1.2),
so drop the NOTREACHED comment.

swapfunc() is not passed to pthread_create, there's no need for it to
have any arguments.

Drop unnecessary cast to makecontext() argument.

Cleanup trailing whitespace.

PR/51890: Ngie Cooper: include and error message fixes.

branches: 1.2.2; 1.2.4;
Go back to the initial context (as tests/lib/libc/sys/t_swapcontext.c does)
after checking pthread_self() didn't change. Otherwise the process exits
outside of atf context.
Should fix "Test case exited normally but failed to create the results file: Results file is empty" reports from atf-run.

branches: 1.1.4; 1.1.12;
setcontext() used to be incompatible with -lpthread since it affected
the TLS pointer, therefore wrecking the pthread environement.

Some ports had _UC_TLSBASE flag or equivalent (_UC_UNIQUE on alpha)
that controlled whether setcontext() would change the TLS pointer.
This change let libpthread override setcontext() with its own version
that unsets _UC_TLSBASE, enabling safe usage of setcontext() with
-lpthread.

We also have the following required changes here:
- rename alpha's _UC_UNIQUE into _UC_TLSBASE
- add _UC_TLSBASE definition in header file for all ports
(powerpc, sh3, sparc and sparc64 lack the implementation for now)
- introduce a libc stub that can be overriden for setcontext()
- modify MD libcs swapcontext() implementations so that they use the
setcontext() libc stub instead of doing a plain system call.

While we are there:
- document various MD _UC_* flags in header file
- add libc and libpthread tests for swapcontext() behavior
(hopefully helpful to spot MD problems introduced with this change)

Future work:
- Deciding whether kernel support or _UC_TLSBASE should be added for
powerpc, sh3, sparc and sparc64 is left to portmasters
sparc64

Approved by core@