increase system-wide filename limit to 255

. move mfs-specific struct, constants to mfs/, so
	  mfs-specific, on-disk format structs and consts are
	  fully isolated from generic structs and functions
	. removes de and readfs utils

nbsd_include/sys/Makefile

@@ -12,7 +12,7 @@ INCS+= ioc_net.h statfs.h
 INCS+=	ansi.h atomic.h \
 	bitops.h bswap.h \
 	cdefs.h cdefs_aout.h cdefs_elf.h ctype_bits.h ctype_inline.h \
-	dir.h dirent.h exec_elf.h \
+	dirent.h exec_elf.h \
 	endian.h errno.h \
 	fcntl.h fd_set.h featuretest.h file.h \
 	float_ieee754.h gcq.h gmon.h hash.h \