Add ptrace(2) backend for NetBSD

Model this code after existing Linux backend and try to keep differences
to minimum.

Features and design choices, compared to the Linux code:
 - capstone dependency as a disassembler
 - no bfd support
 - -D_KERNTYPES for register_t type
 - no clone() support
 - no disable ASLR switch (personality)
 - no libunwind support, there is no port of HP libunwind version
 - no hardware assisted profiling aid support
 - traditional PT_ATTACH in place of 'seize' operation
 - poll(2) detecting of signals, instead of sigtimedwait(2) which
   didn't work for unknown reasons
 - no cross-ABI support (i386 on amd64 host)
 - currently only amd64 and i386 support

Missing or incomplete features are subject for improvement.

Tested with NetBSD/amd64 8.99.23

Makefile

diff --git a/Makefile b/Makefile
index 86e4b47..0e0ae0c 100644
--- a/Makefile
+++ b/Makefile
@@ -137,6 +137,17 @@
     endif
     ARCH_SRCS := $(sort $(wildcard mac/*.c))
     # OS Darwin
+else ifeq ($(OS),NetBSD)
+    ARCH := NETBSD
+
+    ARCH_SRCS := $(sort $(wildcard netbsd/*.c))
+    ARCH_CFLAGS := -std=c11 -I/usr/local/include \
+                   -Wextra -Wno-override-init \
+                   -funroll-loops -D_KERNTYPES
+    ARCH_LDFLAGS := -L/usr/local/include \
+                    -pthread -lcapstone -lrt
+
+    # OS NetBSD
 else
     ARCH := POSIX
 
@@ -246,7 +257,7 @@
   endif
 endif
 
-SUBDIR_ROOTS := linux mac posix libhfuzz libhfcommon libhfnetdriver
+SUBDIR_ROOTS := linux mac netbsd posix libhfuzz libhfcommon libhfnetdriver
 DIRS := . $(shell find $(SUBDIR_ROOTS) -type d)
 CLEAN_PATTERNS := *.o *~ core *.a *.dSYM *.la *.so *.dylib
 SUBDIR_GARBAGE := $(foreach DIR,$(DIRS),$(addprefix $(DIR)/,$(CLEAN_PATTERNS)))
@@ -449,6 +460,16 @@
 mac/arch.o: arch.h honggfuzz.h libhfcommon/util.h fuzz.h libhfcommon/common.h
 mac/arch.o: libhfcommon/files.h libhfcommon/common.h libhfcommon/log.h
 mac/arch.o: sancov.h subproc.h
+netbsd/arch.o: arch.h honggfuzz.h libhfcommon/util.h fuzz.h
+netbsd/arch.o: libhfcommon/common.h libhfcommon/files.h libhfcommon/common.h
+netbsd/arch.o: libhfcommon/log.h libhfcommon/ns.h netbsd/trace.h
+netbsd/arch.o: sancov.h sanitizers.h subproc.h
+netbsd/trace.o: netbsd/trace.h honggfuzz.h libhfcommon/util.h
+netbsd/trace.o: libhfcommon/common.h libhfcommon/files.h libhfcommon/common.h
+netbsd/trace.o: libhfcommon/log.h netbsd/unwind.h sancov.h
+netbsd/trace.o: sanitizers.h socketfuzzer.h subproc.h
+netbsd/unwind.o: netbsd/unwind.h honggfuzz.h libhfcommon/util.h
+netbsd/unwind.o: libhfcommon/common.h libhfcommon/log.h
 posix/arch.o: arch.h honggfuzz.h libhfcommon/util.h fuzz.h
 posix/arch.o: libhfcommon/common.h libhfcommon/files.h libhfcommon/common.h
 posix/arch.o: libhfcommon/log.h sancov.h subproc.h