terminal: trim memory after free:ing a terminal

A terminal with lots of scrollback history will have allocated a lot
of memory.

Normally, free() wont return this memory to the OS, and we don't seem
to trigger the automatic trim calls.

This means the server would accumulate quite a lot of memory over
time, as terminals come and go.

Now we explicitly trim the memory every time a terminal is destroyed.

terminal.c

@@ -1,5 +1,6 @@
 #include "terminal.h"
 
+#include <malloc.h>
 #include <string.h>
 #include <unistd.h>
 #include <assert.h>
@@ -796,6 +797,8 @@ term_destroy(struct terminal *term)
     }
 
     free(term);
+    if (!malloc_trim(0))
+        LOG_WARN("failed to trim memory");
     return ret;
 }