Add NetBSD and DragonFly BSD as host platforms

[zakinko]

Bazel already builds and runs on FreeBSD and OpenBSD; this extends the
same host support to NetBSD and DragonFly BSD, so that a bazel built
from source with scripts/bootstrap runs natively there.

The changes are the same shape as the existing FreeBSD/OpenBSD ones:

• OS.java learns NETBSD and DRAGONFLY and counts them POSIX-compatible;
  AutoCpuConverter, ConfigExpander, ConstraintConstants, JniLoader and
  BazelRuleClassProvider gain the matching arms (default shell, PATH,
  the @platforms//os constraint, the libunix_jni.so load).
• src/conditions gains netbsd and dragonfly config_settings, used by the
  C++ client, the native JNI library, singlejar and the sandbox stub to
  pick the BSD sources the way freebsd/openbsd already do.
• blaze_util_bsd.cc: NetBSD dropped statfs(2), so WarnFilesystemType uses
  statvfs(2), which carries f_fstypename all the same; neither NetBSD nor
  DragonFly offers a way for a process to find its own executable, so both
  fall back to argv[0] like OpenBSD. STANDARD_JAVABASE points at the usual
  JDK location on each.
• unix_jni_bsd.cc: DragonFly's libc defines only the _file extattr
  variants, not the _link ones called here, so HAVE_EXTATTR stays off
  there; sysctlbyname(3) is present. NetBSD leaves both off for now.
• process-tools-darwin.cc: NetBSD exposes process listings through
  KERN_PROC2 and struct kinfo_proc2, whose pid is in p_pid.
• md5.h, singlejar/{diag.h,zip_headers.h,mapped_file_posix.inc}: both are
  <sys/endian.h> / err.h platforms like the other BSDs.
• md5_netbsd.sh doubles md5(1) the way md5_openbsd.sh does, since NetBSD
  has neither md5sum nor GNU coreutils.

DragonFly's config_settings reference @platforms//os:dragonfly, which is
added by bazelbuild/platforms#143; until that is released the DragonFly
selects will not resolve on other platforms. NetBSD needs nothing new
from platforms (os:netbsd already exists). The jni_md_header targets come
from a companion rules_java change (bazelbuild/rules_java#381).

Bootstrapped from source and run on NetBSD 11.0/amd64. Also bootstrapped
and run on DragonFly 6.4 with platforms#143 applied, on which the whole
abseil test suite passes; that host is no longer available to re-verify
the exact form of this change.