Fix distribution of the two HOWTO files.

These HOWTOs prevented the use of make distcheck and they were not
included in the tarball.

• src/Makefile.am (gpg4win_howto_smime_ll): New.

(HOWTO_files): Set new variable. Use as dependency for all
installers.
(CLEANFILES): add Howto files.

• src/gpg4win.mk.in (HOWTO_template): New. Expand that.
• src/inst-gpg4win.nsi:
• doc/Makefile.am (EXTRA_DIST): Add howtos.
• doc/HOWTO-SMIME.de.txt: Reformat and suggest sha1sum instead of

openssl.

• doc/HOWTO-SMIME.en.txt: Ditto.