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

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

Fix the visibility of several prototypes. Also move pthread_kill() and
pthread_sigmask() to signal.h. In principle, this shouldn't break anything,
since they're already in signal.h on other systems, and the FreeBSD
manpage says that both pthread.h and signal.h need to be included to
get these functions.

Add a hack to declare pthread_t in the P1003.1-2008 namespace
in signal.h.

According to:
http://www.opengroup.org/onlinepubs/009695399/basedefs/sys/types.h.html

#include <sys/types.h>
should include the definitions of pthread types.

PR: standards/78907
Reported by: Brooks Davis
Approved by: das (mentor)