optimize(Packages): optimize reading large files

Using less syscalls and processing data in memory
makes getNumStringsImpl ~300% faster.

The code is much simpler.

src/detection/packages/packages_linux.c

@@ -42,26 +42,19 @@ static uint32_t getNumElements(FFstrbuf* baseDir, const char* dirname, unsigned
 
 static uint32_t getNumStringsImpl(const char* filename, const char* needle)
 {
-    FILE* file = fopen(filename, "r");
-    if(file == NULL)
+    FF_STRBUF_AUTO_DESTROY content = ffStrbufCreate();
+    if (!ffReadFileBuffer(filename, &content))
         return 0;
 
     uint32_t count = 0;
-
-    char* line = NULL;
-    size_t len = 0;
-
-    while(getline(&line, &len, file) != EOF)
+    char *iter = content.chars;
+    size_t needleLength = strlen(needle);
+    while ((iter = memmem(iter, content.length - (size_t)(iter - content.chars), needle, needleLength)) != NULL)
     {
-        if(strstr(line, needle) != NULL)
-            ++count;
+        ++count;
+        iter += needleLength;
     }
 
-    if(line != NULL)
-        free(line);
-
-    fclose(file);
-
     return count;
 }