build-aux/msys-to-w32: simplify the initial interface.

* build-aux/msys-to-w32: simplify the initial over-engineered
interface, and the implementation.
* Makefile.in (epaths-force-w32): Update for the above.
Makefile.in (epaths-force-w32): Simplify.

* Makefile.in (epaths-force-w32): Simplify, assuming that the shell is
bash (as is the case in MSYS).
(msys_w32prefix_subst, msys_sed_sh_escape): Remove (no longer used).
build-aux/msys-to-w32: Minor doc fixes.
Make rmail-epa-decrypt handle more ways of formatting the message.

* mail/rmail.el (rmail-epa-decrypt): Detect armor with line prefixes.
Check more carefully for mime-part specified character set.
Check for mime-part Content Transfer Encoding.
Notify if no armor found.