opts: use xstrdup () to assign char * options

With the previous change to dynamically allocate memory for each
possible configuration source (three different configuration files, CLI,
RPC) the char * options can no longer directly point to the character
strings extracted by getopt() as the memory might be free'd at some
point. This introduces a macro to set the char * options which first
does a xfree() and then a xstrdup().

Signed-off-by: Adrian Reber <areber@redhat.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>