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/dns.c b/src/dns.c
index 90efdad..4f7ed4b 100644
--- a/src/dns.c
+++ b/src/dns.c
@@ -137,7 +137,7 @@
 	struct dns_resolvers *res;
 
 	list_for_each_entry(res, &dns_resolvers, list) {
-		if (!strcmp(res->id, id))
+		if (strcmp(res->id, id) == 0)
 			return res;
 	}
 	return NULL;
@@ -164,7 +164,7 @@
 	struct dns_srvrq *srvrq;
 
 	list_for_each_entry(srvrq, &dns_srvrq_list, list) {
-		if (srvrq->proxy == px && !strcmp(srvrq->name, name))
+		if (srvrq->proxy == px && strcmp(srvrq->name, name) == 0)
 			return srvrq;
 	}
 	return NULL;
@@ -2499,7 +2499,7 @@
 				       mod->counters, mod->counters_size);
 
 				/* Store the ns counters pointer */
-				if (!strcmp(mod->name, "dns")) {
+				if (strcmp(mod->name, "dns") == 0) {
 					ns->counters = (struct dns_counters *)ns->extra_counters->data + mod->counters_off[COUNTERS_DNS];
 					ns->counters->id = ns->id;
 				}