Adjust pgindent script to use pg_bsd_indent 2.0.

Update version-checking code and list of switches.  Delete obsolete
quasi-support for using GNU indent.  Remove a lot of no-longer-needed
workarounds for bugs of the old version, and improve comments for
the hacks that remain.  Update run_build() subroutine to fetch the
pg_bsd_indent code from the newly established git repo for it.

In passing, fix pgindent to not overwrite files that require no changes;
this makes it a bit more friendly to run on a built tree.

Adjust relevant documentation.

Remove indent.bsd.patch; it's not relevant anymore (and was obsolete
long ago anyway).  Likewise remove pgcppindent, since we're no longer
in the business of shipping C++ code.

Piotr Stefaniak is responsible for most of the algorithmic changes
to the pgindent script; I did the rest.

Discussion: https://postgr.es/m/E1dAmxK-0006EE-1r@gemulon.postgresql.org
Discussion: https://postgr.es/m/30527.1495162840@sss.pgh.pa.us

Author: tglsfdc

src/tools/pgindent/README

@@ -10,7 +10,9 @@ http://adpgtech.blogspot.com/2015/05/running-pgindent-on-non-core-code-or.html
 
 PREREQUISITES:
 
-1) Install pg_bsd_indent in your PATH (see below for details).
+1) Install pg_bsd_indent in your PATH.  Fetch its source code with
+   git clone https://git.postgresql.org/git/pg_bsd_indent.git
+   then follow the directions in README.pg_bsd_indent therein.
 
 2) Install entab (src/tools/entab/).
 
@@ -57,6 +59,9 @@ VALIDATION:
 
    Your configure switches should include at least --enable-tap-tests
    or else much of the Perl code won't get exercised.
+   The ecpg regression tests may well fail due to pgindent's updates of
+   header files that get copied into ecpg output; if so, adjust the
+   expected-files to match.
 
 3) If you have the patience, it's worth eyeballing the "git diff" output
    for any egregiously ugly changes.  See below for cleanup ideas.
@@ -100,10 +105,10 @@ you're at it.
 BSD indent
 ----------
 
-We have standardized on NetBSD's indent, and renamed it pg_bsd_indent.
-We have fixed a few bugs which requre the NetBSD source to be patched
-with indent.bsd.patch patch.  A fully patched version is available at
-https://ftp.postgresql.org/pub/dev.
+We have standardized on FreeBSD's indent, and renamed it pg_bsd_indent.
+pg_bsd_indent does differ slightly from FreeBSD's version, mostly in
+being more easily portable to non-BSD platforms.  You can obtain it from
+https://git.postgresql.org/git/pg_bsd_indent.git
 
 GNU indent, version 2.2.6, has several problems, and is not recommended.
 These bugs become pretty major when you are doing >500k lines of code.
@@ -132,8 +137,10 @@ src/backend/utils/fmgrtab.c is excluded because it confuses pgindent
 and it's a derived file anyway.
 
 src/interfaces/ecpg/test/expected/ is excluded to avoid breaking the ecpg
-regression tests.  Several *.h files are included in regression output so
-they must not be changed.
+regression tests, since what ecpg generates is not necessarily formatted
+as pgindent would do it.  (Note that we do not exclude ecpg's header files
+from the run; some of them get copied verbatim into ecpg's output, meaning
+that the expected files may need to be updated to match.)
 
 src/include/snowball/libstemmer/ and src/backend/snowball/libstemmer/
 are excluded because those files are imported from an external project,