include, libctf: header and soname changes for CTFv4

These changes bump the current file format version to CTF_VERSION_4, and
introduce a new VERSION_5 identical with it to get the version integer and
the name identical again.  A great many changes are made to account for
the changes to handle CTFv4 (which is a BTF superset).

libctf will not compile after these changes, which is why it's been diked
out of the build system and forced-off until the series is complete.

Because all the CTF_K constants have changed values, this is necessarily an
ABI break: add a #define to make picking up this break at compile time
obvious.

Note that the ABI has broken by bumping the soname (deriving it now from
libctf/libtool-version) and folding all newer symbols in the symbol version
file into a new LIBCTF_2.0 version, which is now the only exported version.

libctf/libtool-version

@@ -0,0 +1,30 @@
+# This file is used to maintain libtool version info for libctf.  See
+# the libtool manual to understand the meaning of the fields.  This is
+# a separate file so that version updates don't involve re-running
+# automake.
+#
+# Here are a set of rules to help you update your library version
+# information:
+#
+# 1. Start with version information of `0:0:0' for each libtool library.
+#
+# 2. Update the version information only immediately before a public
+#    release of your software. More frequent updates are unnecessary,
+#    and only guarantee that the current interface number gets larger
+#    faster.
+#
+# 3. If the library source code has changed at all since the last
+#    update, then increment revision (`c:r:a' becomes `c:r+1:a').
+#
+# 4. If any interfaces have been added, removed, or changed since the
+#    last update, increment current, and set revision to 0.
+#
+# 5. Increase the age value only if the changes made to the ABI are backward
+#    compatible.
+#    a. If any interfaces have been added since the last public release,
+#       then increment age.
+#    b. If any interfaces have been removed since the last public release,
+#       then set age to 0.
+#
+# CURRENT:REVISION:AGE
+1:0:0