agent: New option --s2k-count.

* agent/agent.h (opt): New field 's2k_count'.
* agent/gpg-agent.c (oS2KCount): New enum value.
(opts): New option --s2k-count.
(parse_rereadable_options): Set opt.s2k_count.

This option is useful to speed up the starting of gpg-agent and in
cases where the auto-calibration runs into problems due to a broken
time measurement facility.

• Signed-off-by: Werner Koch <wk@gnupg.org>