Set timezone for the server environment.

Introduced a function `set_timezone' to set local timezone(`TZ')
in the server environment. This is imperative to log accurate
timestamps. Because DNS servers run inside a chroot(2) jail,
file `/etc/localtime' can not be used to know local timezone
information.

Numerous - <time.h> - routines access timezone information from the
`TZ' environment variable. If `TZ' is not set, `/etc/localtime' is
used for the same.