cli: convert "notmuch show" to use the new argument parser

cli: convert "notmuch show" to use the new argument parser

Use the new notmuch argument parser to handle arguments in "notmuch
show". There are three minor functional changes:

1) Also set params.raw = TRUE when defaulting to raw format when part
   is requested but format is not specified. This was a bug, and
   --part=0 without --format=raw did not work previously.

2) Set params.decrypt = FALSE if crypto context creation fails.

3) Only use the parameters for the last --format if specified multiple
   times. Previously this could have resulted in a non-working mixture
   of parameters.

Signed-off-by: Jani Nikula <jani@nikula.org>