install-font.sh -P needs to allow dots in its argument.

[T. Kurt Bond]

The linux version of homebrew installs groff's data directory in
/home/linuxbrew/.linuxbrew/share/groff by default.

I must have cargo culted the check_optarg without checking everything out
when I added the -P option.  I've attached a patch to install-font.sh to
make it check that the argument to -P is a directory and that it is
writable.

I kept the check that the argument doesn't start with a dash, even though
someone evil might start their directory with a dash.  Should that check be
dropped?

-- 
T. Kurt Bond, tkurtbond@gmail.com, https://tkurtbond.github.io

install-font.patch
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