Changes to open-file caching limits and uncached file unserialization.

- Unserializing files that were previously kicked out of the open-file
  cache would cause them to be fopen'd with the original access
  permissions which is usually 'w' and causes truncation.  They
  are now opened in 'a' mode.  (addresses #780)

- Add 'max_files_in_cache' script option to manually set the maximum
  amount of opened files to keep cached.  Mainly this just helped
  to create a simple test case for the above change.

- Remove unused NO_HAVE_SETRLIMIT preprocessor switch.

- On systems that don't enforce a limit on number of files opened for
  the process, raise default max size of open-file cache from
  32 to 512.

scripts/base/init-bare.bro

@@ -2329,6 +2329,11 @@ type bt_tracker_headers: table[string] of string;
 ## BPF filter the user has set via the -f command line options. Empty if none.
 const cmd_line_bpf_filter = "" &redef;
 
+## The maximum number of open files to keep cached at a given time.
+## If set to zero, this is automatically determined by inspecting
+## the current/maximum limit on open files for the process.
+const max_files_in_cache = 0 &redef;
+
 ## Deprecated.
 const log_rotate_interval = 0 sec &redef;