Add timeout for dump_file.

It turns out dump_file is used on a number of /proc and system files.
In one case, the read of a file stalled and caused a bugreport to
hang forever. It's still possible if there is a kernel bug that this
could stall forever, but less likely.

Also, change the return type of nanotime to uint64_t.

Testing:
- Created a named fifo and verified that dump_file fails with a timeout.
- Created a large /data/anr/traces.txt to verify that large files still
  dump properly and that the additional NONBLOCK parameter doesn't cause
  a problem.
- Created a dummy /data/tombstones/tombstone_00 to verify that the
  dump of these files still works.
- Compared a dump using the old dumpstate to the new dumpstate to verify
  nothing obviously different.

Bug: 19117030
Change-Id: I0d3dd27583c853cdaccd2fd278748cb5f9ccd4fb