Block allocation map is now built in a separate thread, and does not delay

server startup (sparse write avoidance doesn't happen until it is finished).
Added mutex to bitset functions, which were already being called from
multiple threads.  Rewrote allocation map builder to request file
information in multiple chunks, to avoid uninterruptible wait and dynamic
memory allocation.

src/ioutil.h

@@ -4,13 +4,13 @@
 #include "serve.h"
 struct bitset_mapping; /* don't need whole of bitset.h here */
 
-/** Returns a bit field representing which blocks are allocated in file
-  * descriptor ''fd''.  You must supply the size, and the resolution at which
-  * you want the bits to represent allocated blocks.  If the OS represents
-  * allocated blocks at a finer resolution than you've asked for, any block
-  * or part block will count as "allocated" with the corresponding bit set.
+/** Scan the file opened in ''fd'', set bits in ''allocation_map''  that
+  * correspond to which blocks are physically allocated on disc (or part-
+  * allocated).  If the OS represents allocated blocks at a finer resolution 
+  * than you've asked for, any block or part block will count as "allocated" 
+  * with the corresponding bit set.  Returns 1 if successful, 0 otherwise.
   */
-struct bitset_mapping* build_allocation_map(int fd, off64_t size, int resolution);
+int build_allocation_map(struct bitset_mapping* allocation_map, int fd);
 
 /** Repeat a write() operation that succeeds partially until ''size'' bytes
   * are written, or an error is returned, when it returns -1 as usual.