Improved handling of uprobes.ko by compile-server and client.

Improved handling of uprobes.ko by compile-server and client.

- Server now returns uprobes.ko to client when required.
- New optional argument to staprun's -u option to specify the location
  of uprobes module.
- Client uses new argument of -u to specify uprobes.ko returned by
  the server.
- staprun still prefers already-loaded uprobes, built-in uprobes and
  kernel-built uprobes over the one returned by the server.
- staprun verifies the signature of the uprobes module for unprivileged
  users.