Makefile fix and release version upgrade

This commit is a small milestone -- here comes the second stable release
version of smime-gate (named 1.2).

* Makefile
When doing 'make install', don't overwrite configuration files, if they
already exist.
BUGFIX: mail buffer extend algorithm and encryption algorithm

* src/smime-tool
Changed default encryption (symmetric) algorithm to AES128 -- faster and
safer then RC2.

* src/smime.c
Small change, big difference. Extending buffer by doubling its size
gives much better bandwidth (for messages >90KB).