roffit: Decrease the indent level for .PP

[jay]

Prior to this change when .PP (regular paragraph) was used after .IP (indented paragraph) roffit would not change the indent level which resulted in regular paragraphs having the same indent as the previously indented paragraph.

Ref: curl/curl#13803 (comment)

Closes #xxxxx

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I can't figure out if we should reset it like $indentlevel = 0 or just decrease it but in testing just decreasing seems to work as expected.

[jay]

according to groff manpage for .PP the "left margin and indentation are reset to default values" so I'm thinking the level should be reset to 0

[bagder]

Not sure the distinction will make a big difference because these instructions never increase indent level much.

But I noticed in experimenting with this that .RE / .RS actually has a separate indentation level that .PP actually does not reset. We can deal with that later if we run into issues with it.

An nroff sequence like this:

.IP p.roffit
Used for the advertising final paragraph.
.RS
moo

.PP
Hello

Causes this output:

       p.roffit
              Used for the advertising final paragraph.
              moo

              Hello

The input without the .RS:

.IP p.roffit
Used for the advertising final paragraph.

moo

.PP
Hello

... then makes .PP go back correctly;

       p.roffit
              Used for the advertising final paragraph.

              moo

       Hello