Fix error.h non-static variables for liblttng-ctl

Linking with liblttng-ctl made the variable opt_quiet and opt_verbose
undefined if nonexistent in the linked application.

Rename the variables adding the prefix lttng_* and declaring them in
liblttng-ctl as global variable. The user can now control the verbosity
of the library by simply setting them.

Future work will mostly add an API call to control verbosity.

(closes #151)

Signed-off-by: David Goulet <dgoulet@efficios.com>