mksh/misc.h: hack for bsd_signal()

For some reason, I wasn't able to get bsd_signal to be exposed by
signal.h, despite using _XOPEN_SOURCE=600 and _GNU_SOURCE/-std=gnu++98.

This hack is close enough. From the man page for bsd_signal(3):

   On modern Linux systems, bsd_signal() and signal(2) are equivalent.
   But on older systems, signal(2) provided unreliable signal semantics;
   see signal(2) for details.

include/mksh/misc.h

@@ -51,5 +51,6 @@ extern void	warning_mksh(char * message, ...);
 extern void	append_string(register char *from, register String to, register int length);
 extern wchar_t	*get_wstring(char * from);
 
+#define bsd_signal(x, y) signal(x, y)
 
 #endif