Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

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

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Implement sighold, sigignore, sigpause, sigrelse, sigset functions
from SUSv4 XSI. Note that the functions are obsoleted, and only
provided to ease porting from System V-like systems. Since sigpause
already exists in compat with different interface, XSI sigpause is
named xsi_sigpause.

Reviewed by: davidxu
MFC after: 3 weeks

Use C comments since we now preprocess these files with CPP.

Add symbol maps and initial symbol version definitions to libc.

Reviewed by: davidxu