as per http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=242638
Noted use of _XOPEN_SOURCE to get macros <sytdlib.h>
as for wait(2).
mtk
Changed name of argument from 'string' to 'command' (like POSIX).
Noted that glibc does nowadays explicitly check for the existence
of the shell if 'command' is NULL, rather than the older behaviour
of assuming the shell exists and always returning 1 if
'command' is NULL.
Other wording and formatting clean-ups.