gdbserver: fix handling of single quote arguments

gdbserver: fix handling of single quote arguments

I noticed that passing arguments containing single quotes to gdbserver
didn't work correctly:

  gdb -ex 'set sysroot' --args /tmp/show-args
  Reading symbols from /tmp/show-args...
  (gdb) target extended-remote | gdbserver --once --multi - /tmp/show-args
  Remote debugging using | gdbserver --once --multi - /tmp/show-args
  stdin/stdout redirected
  Process /tmp/show-args created; pid = 176054
  Remote debugging using stdio
  Reading symbols from /lib64/ld-linux-x86-64.so.2...
  (No debugging symbols found in /lib64/ld-linux-x86-64.so.2)
  0x00007ffff7fd3110 in _start () from /lib64/ld-linux-x86-64.so.2
  (gdb) set args \'
  (gdb) r
  The program being debugged has been started already.
  Start it from the beginning? (y or n) y
  Starting program: /tmp/show-args \'
  stdin/stdout redirected
  Process /tmp/show-args created; pid = 176088
  2 args are:
    /tmp/show-args
    \'
  Done.
  [Inferior 1 (process 176088) exited normally]
  (gdb) target native
  Done.  Use the "run" command to start a process.
  (gdb) run
  Starting program: /tmp/show-args \'
  2 args are:
    /tmp/show-args
    '
  Done.
  [Inferior 1 (process 176095) exited normally]
  (gdb) q

The 'shows-args' program used here just prints the arguments passed to
the inferior.

Notice that when starting the inferior using the extended-remote
target the second argument is "\'", while when running using native
target the argument is "'".  The second of these is correct, the \'
used with the "set args" command is just to show GDB that the single
quote is not opening an argument string.

It turns out that the extra backslash is injected on the gdbserver
side when gdbserver processes the arguments that GDB passes it, the
code that does this was added as part of this much larger commit:

  commit 2090129c36c7e582943b7d300968d19b46160d84
  Date:   Thu Dec 22 21:11:11 2016 -0500

      Share fork_inferior et al with gdbserver

In this commit I propose removing the specific code that adds what I
believe is a stray backslash.  I've extended an existing test to cover
this case, and I now see identical behaviour when using an
extended-remote target as with the native target.

This partially fixes PR gdb/27989, though there are still some issues
with newline handling which I'll address in a later commit.

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>