Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Don't use an empty struct.

Move MINSIGSTKSZ from <machine/signal.h> to <machine/_limits.h> and rename
it to __MINSIGSTKSZ. Define MINSIGSTKSZ in <sys/signal.h>.

This is done in order to use MINSIGSTKSZ for the macro PTHREAD_STACK_MIN
in <pthread.h> (soon <limits.h>) without having to include the whole
<sys/signal.h> header.

Discussed with: bde

Start all license statements with /*-

Remove the empty definition of struct osigcontext, as it will never be used.

Don't declare osigset_t, as it is done in sys/_sigset.h.

Remove advertising clause from University of California Regent's license,
per letter dated July 22, 1999.

Approved by: core

Add standards visibility conditionals. Change any uses of sigset_t to
struct __sigset to avoid depending on objects from <sys/signal.h>.

We need machine/{signal,ucontext}.h to build a cross GCC compiler.
So craft the proper versions of these and commit em.