email plugin: Simplified code.

email plugin: Simplified code.

While looking at the code for some reason, I decided to simplify and
improve large parts of it. Most notably, standard IO streams are now used
to read from the socket. This allowed to remove large parts of the code
which were used to read and buffer data from the socket so far.

Also among the changes:
 * free any allocated memory
 * added / improved log messages
 * do not require euid == 0 to chown() the socket

Signed-off-by: Sebastian Harl <sh@tokkee.org>
Signed-off-by: Florian Forster <octo@huhu.verplant.org>