Fixed wrong prototype and missing include for strsignal(3). strsignal()

Fixed wrong prototype and missing include for strsignal(3).  strsignal()
takes an int arg and is prototyped in <string.h>.  It has the opposite
interface botches to psignal(3) which takes a bogus unsigned arg but is
prototyped in the right place.

This is not the last of the interface problems for strsignal().  We
obtained it from NetBSD, but NetBSD has moved its prototype to
<unistd.h>.  strsignal() should return const char *, but it returns
char * for historical reasons.  NetBSD declares it as returning
__aconst char, where __aconst is normally empty but can be set to
`const' to give better error checking.  glibc-2.1.1 prototypes
strsignal() in <string.h>.