linux-user: Clean up arg_start/arg_end confusion

linux-user: Clean up arg_start/arg_end confusion

We had two sets of variables: arg_start/arg_end, and
arg_strings/env_strings.  In linuxload.c, we set the
first pair to the bounds of the argv strings, but in
elfload.c, we set the first pair to the bounds of the
argv pointers and the second pair to the bounds of
the argv strings.

Remove arg_start/arg_end, replacing them with the standard
argc/argv/envc/envp values.  Retain arg_strings/env_strings
with the meaning we were using in elfload.c.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/714
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220427025129.160184-1-richard.henderson@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>