CLEANUP: Compare the return value of `XXXcmp()` functions with zero

According to coding-style.txt it is recommended to use:

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

So let's do this.

The change was performed by running the following (very long) coccinelle patch
on src/:

    @@
    statement S;
    expression E;
    expression F;
    @@

      if (
    (
    dns_hostname_cmp
    |
    eb_memcmp
    |
    memcmp
    |
    strcasecmp
    |
    strcmp
    |
    strncasecmp
    |
    strncmp
    )
    -  (E, F)
    +  (E, F) != 0
      )
    (
      S
    |
      { ... }
    )

    @@
    statement S;
    expression E;
    expression F;
    @@

      if (
    - !
    (
    dns_hostname_cmp
    |
    eb_memcmp
    |
    memcmp
    |
    strcasecmp
    |
    strcmp
    |
    strncasecmp
    |
    strncmp
    )
    -  (E, F)
    +  (E, F) == 0
      )
    (
      S
    |
      { ... }
    )

    @@
    expression E;
    expression F;
    expression G;
    @@

    (
    G &&
    (
    dns_hostname_cmp
    |
    eb_memcmp
    |
    memcmp
    |
    strcasecmp
    |
    strcmp
    |
    strncasecmp
    |
    strncmp
    )
    -  (E, F)
    +  (E, F) != 0
    )

    @@
    expression E;
    expression F;
    expression G;
    @@

    (
    G ||
    (
    dns_hostname_cmp
    |
    eb_memcmp
    |
    memcmp
    |
    strcasecmp
    |
    strcmp
    |
    strncasecmp
    |
    strncmp
    )
    -  (E, F)
    +  (E, F) != 0
    )

    @@
    expression E;
    expression F;
    expression G;
    @@

    (
    (
    dns_hostname_cmp
    |
    eb_memcmp
    |
    memcmp
    |
    strcasecmp
    |
    strcmp
    |
    strncasecmp
    |
    strncmp
    )
    -  (E, F)
    +  (E, F) != 0
    && G
    )

    @@
    expression E;
    expression F;
    expression G;
    @@

    (
    (
    dns_hostname_cmp
    |
    eb_memcmp
    |
    memcmp
    |
    strcasecmp
    |
    strcmp
    |
    strncasecmp
    |
    strncmp
    )
    -  (E, F)
    +  (E, F) != 0
    || G
    )

    @@
    expression E;
    expression F;
    expression G;
    @@

    (
    G &&
    - !
    (
    dns_hostname_cmp
    |
    eb_memcmp
    |
    memcmp
    |
    strcasecmp
    |
    strcmp
    |
    strncasecmp
    |
    strncmp
    )
    -  (E, F)
    +  (E, F) == 0
    )

    @@
    expression E;
    expression F;
    expression G;
    @@

    (
    G ||
    - !
    (
    dns_hostname_cmp
    |
    eb_memcmp
    |
    memcmp
    |
    strcasecmp
    |
    strcmp
    |
    strncasecmp
    |
    strncmp
    )
    -  (E, F)
    +  (E, F) == 0
    )

    @@
    expression E;
    expression F;
    expression G;
    @@

    (
    - !
    (
    dns_hostname_cmp
    |
    eb_memcmp
    |
    memcmp
    |
    strcasecmp
    |
    strcmp
    |
    strncasecmp
    |
    strncmp
    )
    -  (E, F)
    +  (E, F) == 0
    && G
    )

    @@
    expression E;
    expression F;
    expression G;
    @@

    (
    - !
    (
    dns_hostname_cmp
    |
    eb_memcmp
    |
    memcmp
    |
    strcasecmp
    |
    strcmp
    |
    strncasecmp
    |
    strncmp
    )
    -  (E, F)
    +  (E, F) == 0
    || G
    )

    @@
    expression E;
    expression F;
    expression G;
    @@

    (
    - !
    (
    dns_hostname_cmp
    |
    eb_memcmp
    |
    memcmp
    |
    strcasecmp
    |
    strcmp
    |
    strncasecmp
    |
    strncmp
    )
    -  (E, F)
    +  (E, F) == 0
    )
diff --git a/src/compression.c b/src/compression.c
index e3f34b5..785aa48 100644
--- a/src/compression.c
+++ b/src/compression.c
@@ -131,7 +131,7 @@
 	int i;
 
 	for (i = 0; comp_algos[i].cfg_name; i++) {
-		if (!strcmp(algo, comp_algos[i].cfg_name)) {
+		if (strcmp(algo, comp_algos[i].cfg_name) == 0) {
 			comp_algo = calloc(1, sizeof(*comp_algo));
 			memmove(comp_algo, &comp_algos[i], sizeof(struct comp_algo));
 			comp_algo->next = comp->algos;