gdbserver: handle newlines in inferior arguments

Similarly to how single quotes were mishandled, which was fixed two
commits ago, this commit fixes handling of newlines in arguments
passed to gdbserver.

We already had a test that covered this, gdb.base/args.exp, which,
when run with the native-extended-gdbserver board contained several
KFAIL covering this situation.

In this commit I remove the unnecessary, attempt to quote incoming
newlines within arguments, and do some minimal cleanup of the related
code.  There is additional cleanup that can be done, but I'm leaving
that for the next commit.

Then I've removed the KFAIL from the test case, and performed some
minimal cleanup there too.

After this commit the gdb.base/args.exp is 100% passing with the
native-extended-gdbserver board file.

During review I was pointed to this older series:

  https://inbox.sourceware.org/gdb-patches/20211022071933.3478427-1-m.weghorn@posteo.de/

which also includes this fix as part of a larger set of changes.  I'm
giving a Co-Authored-By credit to the author of that original series.
I believe this smaller fix brings some benefits on its own, though the
original series does offer additional improvements.  Once this is
merged I'll take a look at rebasing and resubmitting the original series.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=27989



Co-Authored-By: Michael Weghorn <m.weghorn@posteo.de>
Approved-By: Tom Tromey <tom@tromey.com>