Move fts.h and functions from libroot to libbsd.

These are BSD extensions, not POSIX functions. They were needed
in libroot by the previous versions of the ftw/nftw implementations,
but the musl versions do not need them, and so we can move them to
libbsd.

This is a minor ABI break, but hopefully whatever was using them
in libroot also links to libbsd. If not, that's an easy enough fix.
(These were only added to libroot in 2013.)

headers/compatibility/bsd/fts.h

@@ -33,6 +33,10 @@
 #ifndef	_FTS_H_
 #define	_FTS_H_
 
+#include <features.h>
+
+#ifdef _DEFAULT_SOURCE
+
 #include <sys/cdefs.h>
 
 typedef struct {
@@ -135,4 +139,6 @@ int	 fts_set(FTS *, FTSENT *, int);
 void	 fts_set_clientptr(FTS *, void *);
 __END_DECLS
 
+#endif /* _DEFAULT_SOURCE */
+
 #endif /* !_FTS_H_ */

src/libs/bsd/Jamfile

@@ -13,6 +13,7 @@ for architectureObject in [ MultiArchSubDirSetup ] {
 			err.c
 			explicit_bzero.c
 			fgetln.c
+			fts.c
 			getpass.c
 			issetugid.c
 			lutimes.c

src/system/libroot/posix/Jamfile

@@ -32,7 +32,6 @@ for architectureObject in [ MultiArchSubDirSetup ] {
 			dirent.c
 			errno.c
 			fcntl.cpp
-			fts.c
 			glob.c
 			inttypes.c
 			libgen.cpp