Require /bin/sh support XSI extensions for kill and trap

Require /bin/sh support XSI extensions for kill and trap

Due to the widespread use of these extensions by scripts already in
the archive, require that /bin/sh support the XSI extensions for kill
and trap which permit kill -<signal> and use of numeric signals for
well-known signals.  Also permit trapping of SIGPIPE by signal number,
since this is done by well-known portable shell scripts like libtool.