check_pgsql: Added support for executing queries.

The query result (the double value of the first column in the first row, to be
precise) will be checked against threshold ranges specified using the -C and
-W options.

Note that this also allows to query PostgreSQL internal values using the
information available from the database daemon's "statistics collector" -- see
the chapter "Monitoring Database Activity" in the PostgreSQL manual for
details.
check_pgsql: Fixed query perfdata output for empty warn/crit ranges.

Previously, "(null)" was printed (when using GNU's libc). This has been
changed to print the empty string instead.
check_pgsql: Use PQconnectdb() rather than PQsetdbLogin().

This is more flexible and the recommended way to connect to a PostgreSQL
database.

Also, the verbose output now includes detailed information about the
connection.
check_pgsql: Allow UNIX socket directories as hostname as well.

PostgreSQL accepts the directory name of its UNIX socket as hostname as well,
e.g. /var/run/postgresql/.
check_pgsql: Removed -4/-6 flags from help output.

These options are not currently supported.
check_pgsql: Added support for the -o command line option.

This option may be used to specify further connection parameters to be passed
to PQconnectdb(). For example, this may be used to specify a service name in
pg_service.conf to be used for additional connection parameters: -o
'service=<name>' or to specify the SSL mode: -o 'sslmode=require'.

See the chapter "libpq - C Library" in the PostgreSQL manual for details.
check_pgsql: Updated copyright.
check_pgsql: Leave 'min' value in query perfdata empty.

There is no reasonable default value for that.
check_pgsql: Determine connection time in µs-resolution.

… thus, treat "elapsed time" and the thresholds as floating point values.
Adding tests for range @1:1
plugins/t/check_apt.t: Expect performance data

The check_apt plugin now emits performance data, fix the test(s)
accordingly.
plugins/t/check_procs.t: Expect performance data

The check_procs plugin now emits performance data, fix the test(s)
accordingly.
Merge remote-tracking branch 'github/tokkee/sh/check_pgsql'

* github/tokkee/sh/check_pgsql:
  check_pgsql: Determine connection time in µs-resolution.
  check_pgsql: Leave 'min' value in query perfdata empty.
  check_pgsql: Updated copyright.
  check_pgsql: Added support for the -o command line option.
  check_pgsql: Removed -4/-6 flags from help output.
  check_pgsql: Allow UNIX socket directories as hostname as well.
  check_pgsql: Use PQconnectdb() rather than PQsetdbLogin().
  check_pgsql: Fixed query perfdata output for empty warn/crit ranges.
  check_pgsql: Added support for executing queries.

Conflicts:
	plugins/check_pgsql.c
check_pgsql: Remove -4/-6 flags from help output

The -4/-6 options are currently not supported.
NEWS: Mention the new check_pgsql features

Closes #16.