Add support for NetBSD as a host OS

With these changes, I am able to run Manticore on Linux (static)
binaries from a NetBSD host.

Tested with NetBSD/amd64 (7.1_STABLE XEN3_DOM0).

manticore/utils/mappings.py

@@ -43,5 +43,38 @@ if osname == "darwin" or osname.startswith("linux"):
     mmap = _mmap
     munmap = _munmap
 
+elif osname.startswith("netbsd"):
+
+    libc = ctypes.cdll.LoadLibrary("libc.so.12")
+
+    # void* mmap(void* addr, size_t len, int prot, int flags, int fd, off_t offset)
+    mmap_function = libc.mmap
+    mmap_function.restype = ctypes.c_void_p
+    mmap_function.argtype = [ctypes.c_void_p, ctypes.c_size_t,
+                             ctypes.c_int, ctypes.c_int, ctypes.c_int, ctypes.c_size_t]
+    # int munmap(void* addr, size_t len)
+    munmap_function = libc.munmap
+    munmap_function.restype = ctypes.c_int
+    munmap_function.argtype = [ctypes.c_void_p, ctypes.c_size_t]
+
+    def _munmap(address, size):
+        result = munmap_function(address, size)
+        assert result == 0
+
+    def _mmap(fd, offset, size):
+        prot = MMAP.PROT_READ | MMAP.PROT_WRITE
+        flags = MMAP.MAP_PRIVATE
+
+        if size & 0xfff != 0:
+            size = (size & ~0xfff) + 0x1000
+
+        assert size > 0
+
+        result = mmap_function(0, size, prot, flags, fd, offset)
+        return ctypes.cast(result, ctypes.POINTER(ctypes.c_char))
+
+    mmap = _mmap
+    munmap = _munmap
+
 else:
     raise ValueError("Unsupported host OS: " + osname)