Use m_temp instead of mutt_mktemp in compress code

Use m_temp instead of mutt_mktemp in compress code

This patch remove the static set_path() function which was used only twice.
in:
- mutt_open_read_compressed()
  We just create a temporary file and open it, so this is more secure
  against symlink attacks and then just call decompression cmd normaly
- mutt_open_append_compressed()
  Instead of creating the file if (cond), unlink() it if (!cond).

Signed-off-by: Julien Danjou <julien@danjou.info>
Signed-off-by: Pierre Habouzit <madcoder@debian.org>