put old m_snsubst into file.c as m_file_fmt.

put old m_snsubst into file.c as m_file_fmt.
implement m_temp{fd,file} functions to replace mutt_(adv_)mktemp, that
work like that:

  m_tempFOO(dst, dstlen, tmpdir, fmt).

it returns either a fd or a FILE* (check it's properly >= 0 or !NULL !!!)
and put the chose name in the buffer dst of size dstlen.

tmpdir is the directory where to create files.

fmp is a file format, where we strip any directory part, then use the
remains like that:

  - if there is a %s specifier, fmt is used as a simple file format
    specifier, and %s is substituted with a temporary file name.
  - if there is no %s specifier, fmt is supposed to be a file name we
    want to take as model, meaning we just want to copy its extension.
    So if you pass "foo.jpg" as a fmt, the resulting file name will end
    in .jpg

mark mutt_(adv_)?mktemp functions as obsolete, and currently
reimplement them with m_temp* functions as a proof of concept.

Signed-off-by: Pierre Habouzit <madcoder@debian.org>