constify the parser structs

untangle a bit buffers and parsers Now buffers have a nicer interface, taking explicitly the buffer where they push stuff, and the code that deals with buffers doesn't need to peek into the parsers. The parsers now are 'pure' structs without state attached to them, the state was moved to the buffer.

identity: don't use getopt() in main we can't portably reset it.

identity: few fixes Correct export usage; rename variable in cmd_mappings, allow to filter by identity name.

draft of telescope-identity(1) This is intended to generate and manage certificates outside of telescope. Some operations are already available in the telescope UI, such as use and forget a certificate for a specific page, but generate and removal are hard at the moment to implement inside telescope limited UI, so provide an helper program for that.