Add support for NetBSD

[jaredmcneill]

This change adds support for X86 and Arm based NetBSD guests.

A lot of the FreeBSD specific quirks also apply to NetBSD and are reused where possible. A few places where NetBSD differs:

• FUSE compatibility is provided by the librefuse library.
• Minor differences in the xdr(3) APIs.
• NetBSD provides struct statvfs with equivalent functionality of struct statfs.
• A gethostbyname_r implementation is provided by libc, but not exposed by any system headers.
• OS specific mcontext_t layout.
• ctype functions, like Win32, are implemented using lookup tables.
• Machine architecture is reported by hw.machine_arch sysctl.
• Thread ID can be obtained with _lwp_self().
• NetBSD provides a Linux compatible /proc/<pid>
• Different prefixes for disk device names (dk, wd, sd, ld).
• No separate libdl or librt libraries.
• No support for thread specific locales (newlocale, uselocale, etc).