libiberty/buildargv: POSIX behaviour for backslash handling

libiberty/buildargv: POSIX behaviour for backslash handling

GDB makes use of the libiberty function buildargv for splitting the
inferior (program being debugged) argument string in the case where
the inferior is not being started under a shell.

I have recently been working to improve this area of GDB, and have
tracked done some of the unexpected behaviour to the libiberty
function buildargv, and how it handles backslash escapes.

For reference, I've been mostly reading:

  https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html

The issues that I would like to fix are:

  1. Backslashes within single quotes should not be treated as an
  escape, thus: '\a' should split to \a, retaining the backslash.

  2. Backslashes within double quotes should only act as an escape if
  they are immediately before one of the characters $ (dollar),
  ` (backtick), " (double quote), ` (backslash), or \n (newline).  In
  all other cases a backslash should not be treated as an escape
  character.  Thus: "\a" should split to \a, but "\$" should split to
  $.

  3. A backslash-newline sequence should be treated as a line
  continuation, both the backslash and the newline should be removed.

I've updated libiberty and also added some tests.  All the existing
libiberty tests continue to pass, but I'm not sure if there is more
testing that should be done, buildargv is used within lto-wraper.cc,
so maybe there's some testing folk can suggest that I run?

2024-07-16  Andrew Burgess  <aburgess@redhat.com>

libiberty/

	* argv.c (buildargv): Backslashes within single quotes are
	literal, backslashes only escape POSIX defined special characters
	within double quotes, and backslashed newlines should act as line
	continuations.
	* testsuite/test-expandargv.c: Add new tests 7, 8, and 9.