agent: Move a typedef to common and provide parse_pinentry_mode.
* common/agent-opt.c: New. * common/shareddefs.h: New. * common/Makefile.am: Add new files. * agent/agent.h: Include shareddefs.h. (pinentry_mode_t): Factor out to shareddefs.h. * agent/command.c (option_handler): Use parse_pinentry_mode.