Inconsistent CLI: keybase pgp sign does not use the --infile option

As of Keybase version 1.0.0-47, almost all of the PGP commands require an explicit `-i` or `--infile` option.

The one exception is `keybase pgp sign`, which still uses the `[command options] [filename]` format for its input, and does not accept an infile option.

Although not a bug, this inconsistency in the CLI is a bit confusing.

Start at the `keybase pgp sign` CLI entry point and compare its argument handling with the other PGP commands that accept `-i` or `--infile`. The work is done when signing accepts `--infile` consistently while preserving the existing filename behavior as appropriate, with the relevant CLI checks passing.

Written by the indexing model from the issue text.