Standardize `strcmp(a, b) == 0` instead of `!strcmp(a, b)`

`!strcmp(a, b)` always looks like a negation to me (not equal), while
the `== 0` version clearly shows that we're after equality.

Semantic patch:

    @@
    expression a;
    expression b;
    @@

    - !strcmp(a, b)
    + strcmp(a, b) == 0

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I8e05b544c5cbd733e3d3e5b2c3554ee98669d1d5
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1685
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>