libdep plugin: fix bugs in parser and drop escaping

libdep plugin: fix bugs in parser and drop escaping

  PR ld/31906
  * libdep_plugin.c (str2vec): Fix bug where null byte was not copied on memmove during quote handling and escaping, causing repeat of the last character in the last argument.
  Fix buffer overflow in **res when arguments were separated by `\t` instead of ` `.
  Remove handling of the escape character `\`, as it made it impossible to specify paths containing `\`
  -- the implementation merely dropped `\`, and was affected by the memmove bug, so this should not be breaking; just single and double quotes are sufficient to deal with white space and quote characters, there is no need for escaping.
  Handle syntax errors   on unterminated quotes.
  Make the parser linear time instead of   quadratic.