Take an ACCESS SHARE LOCK on the target table, in an initial attempt to prevent concurrent DDL.

This is a first pass at Daniele's suggestion in Issue #8, although it is
definitely still buggy -- it is still possible for another transaction
to get in an AccessExclusive lock and perform DDL either before the
ACCESS SHARE lock is acquired or immediately after it is released.
Switch to using pgut_command() and pgut_execute() for conn2.
Further improvements to concurrent-DDL guard.

Fix table locking so that race conditions don't exist between lock
release in primary conn, and lock acquisition in conn2. Also, have
conn2 be in charge of performing the table swap step, to avoid a
similar race.

Part of work for Issue #8.
Several fixes for concurrent-DDL guard.

 * KILL_COMPETING_LOCKS was using pg_cancel_backend() instead of
   pg_terminate_backend()
 * create kill_ddl() function for canceling+terminating any pending
   unsafe concurrent DDL, i.e. anyone hanging out waiting for
   an ACCESS EXCLUSIVE lock on our table.
 * create lock_access_share() function for reliably obtaining an
   ACCESS SHARE lock on the target table, killing off any queued
   ACCESS EXCLUSIVE lockers in the process via kill_ddl()
 * Avoid deadlock possible before we run:
     CREATE TABLE reorg.table_xxx AS SELECT ... FROM ONLY ...
   by using lock_access_share()
 * Fix a few calls in lock_exclusive() which were forgetting to
   specify the passed-in connection.

These fixes are related to Issue #8. The main thing remaining AFAIK
is to review or fix some of the unlikely-error handling bits;
most of these should be marked with XXX now.
Allow multiple --table options to be specified on the command-line.

Per Issue #18. SimpleStringList code borrowed from pg_dump and a
pending patch to add similar functionality to pg_restore,
clusterdb, vacuumdb, and reindexdb.

The error handling in reorg_one_table() could still be much improved,
so that an error processing a single table doesn't cause pg_reorg to
necessarily bail out and skip further tables, but I'll leave that for
another day.
Improved error handling, particularly when processing multiple tables.

Previously, an error while processing any single table would cause
pg_reorg to cause exit() and bail out. Quick summary of fixes:
 * get rid of pgut_atexit_push() and pgut_atexit_pop() use, since
   we are no longer relying on calling exit() to handle mundane errors
 * remove lock_conn_pid variable; we can just use buffer instead
 * lock_exclusive() and lock_access_share() now return bool instead of
   bailing out on any error
 * ERROR-level ereport() or elog() calls now return WARNING instead,
   to avoid bailing out unnecessarily
 * signature of reorg_cleanup() changed; it no longer needs to take a
   void pointer
 * check return of strdup() for vxid
 * Use pgut_rollback() instead of sending ROLLBACK; command directly

There are still one or two FIXMEs left, including fixing table name
escaping, but I'm committing this much.
Fix bogus use of table name parameters.

Mimic the original code, which used execute_elevel() with params to
pass in table names which are assumed to be quoted already by the user.
Since commit 742380f0429b6 included some of the same changes
already in 6a0af679e14d1d, go ahead and include a few more of
the error cleanup from 6a0af679e14d1d.
Move simple_string_list_size() into pgut.c, with the rest of these string functions.

Also, fix an error with this function not actually using its string_list argument.
Rename argument to simple_string_list_size() for consistency.
More consistent error reporting

This patch is a port of Daniele's commit 0be414ad10c32d from his own fork,
"error_reporting" branch.

reorg_all_database can return an error message: in case of any error different
from "missing schema" return the error and keep processing the other databases
instead of printing and stopping the program.

The output of the program is now something like:

    $ pg_reorg --all
    pg_reorg: reorg database "contrib_regression"
    pg_reorg: reorg database "template1" ... skipped: pg_reorg is not installed in the database
Don't forget to disconnect conn2 when disconnect() is called, otherwise
we leave a connection hanging open for every database processed
via pg_repack -a.
Ignore other pg_repack clients which may be running concurrently for the purposes of SQL_XID_SNAPSHOT.

Use application_name from pg_stat_activity, if available, to identify
other pg_repack clients. Fixes Issue #1.
Support for multiple --tables, as well as Concurrent DDL Guard.

Code merged in, with a few more changes, from the multiple_tables branch.

The multiple --table support and SimpleStringList code is largely
borrowed from pg_dump. (pg_reorg Issue #18).

The concurrent DDL guard is implemented using an auxiliary
database connection (pg_reorg Issue #8) which holds an ACCESS SHARE
lock on the target table while pg_repack conducts the rest of its work.
Downgrade ERROR to WARNING in the case where a table does not have a primary key or not-null unique key.

We want to be able to keep processing further tables if we
encounter this problem on one of many --tables.