Reformat psql's --help=variables output.

The previous format with variable names and descriptions in separate
columns was extremely constraining about the length of the descriptions.
We'd dealt with that in several inconsistent ways over the years,
including letting the lines run over 80 characters, breaking descriptions
into multiple lines, or shoving the description onto a separate line.
But it's been a long time since the output could realistically fit onto
a single screen vertically, so let's just rely even more heavily on the
pager to deal with the vertical distance, and split each entry into two
(or more) lines, in the format

  variable-name
    variable description goes here

Each variable name + description remains a single translatable string,
in hopes of reducing translator confusion; we're just changing the
embedded whitespace.

I failed to resist the temptation to copy-edit one or two of the
descriptions while at it.

Discussion: https://postgr.es/m/2947.1504542679@sss.pgh.pa.us