Provide strdup.
 [rsc] --rw-rw-r-- M 661145 glenda sys 1259 Jan 16 20:48 sys/include/ape/string.h
	/n/sourcesdump/2006/0116/plan9/sys/include/ape/string.h:31,36 - 
	/n/sourcesdump/2006/0117/plan9/sys/include/ape/string.h:31,40
	  extern char *strerror(int);
	  extern size_t strlen(const char *);
	  
	+ #ifdef _BSD_EXTENSION
	+ #include <bsd.h>
	+ #endif
	+ 
	  #ifdef __cplusplus
	  }
	  #endif