Added missing "fall through" comments (ticket #1259).

Found by gcc7 (-Wimplicit-fallthrough).

src/core/ngx_murmurhash.c

diff --git a/src/core/ngx_murmurhash.c b/src/core/ngx_murmurhash.c
index c31e0e0..5ade658 100644
--- a/src/core/ngx_murmurhash.c
+++ b/src/core/ngx_murmurhash.c
@@ -35,8 +35,10 @@
     switch (len) {
     case 3:
         h ^= data[2] << 16;
+        /* fall through */
     case 2:
         h ^= data[1] << 8;
+        /* fall through */
     case 1:
         h ^= data[0];
         h *= 0x5bd1e995;

src/http/ngx_http_parse.c

diff --git a/src/http/ngx_http_parse.c b/src/http/ngx_http_parse.c
index 36220fd..e8e5156 100644
--- a/src/http/ngx_http_parse.c
+++ b/src/http/ngx_http_parse.c
@@ -1396,6 +1396,7 @@
                 goto done;
             case '+':
                 r->plus_in_uri = 1;
+                /* fall through */
             default:
                 state = sw_usual;
                 *u++ = ch;
@@ -1437,6 +1438,7 @@
                 goto done;
             case '+':
                 r->plus_in_uri = 1;
+                /* fall through */
             default:
                 state = sw_usual;
                 *u++ = ch;
@@ -1484,6 +1486,7 @@
                 goto done;
             case '+':
                 r->plus_in_uri = 1;
+                /* fall through */
             default:
                 state = sw_usual;
                 *u++ = ch;

src/os/unix/ngx_process.c

diff --git a/src/os/unix/ngx_process.c b/src/os/unix/ngx_process.c
index dd50b5c..993c032 100644
--- a/src/os/unix/ngx_process.c
+++ b/src/os/unix/ngx_process.c
@@ -413,6 +413,7 @@
                 break;
             }
             ngx_debug_quit = 1;
+            /* fall through */
         case ngx_signal_value(NGX_SHUTDOWN_SIGNAL):
             ngx_quit = 1;
             action = ", shutting down";