secrets: a putsecret CLI program

A putsecret main program with the following:

• -B, -Z options for b64 and zip encodings, also -N or whatev for clear text (in case there is already another encoding)
• by default, read secret from stdin, with prompt and hidden input and twice if stdin isatty
• re-use previously existing encoding, if any (must be implemented in the putsecret() library function)
• only per explicit option read a secret from the command line (-s or --secret=)
• keep the order of lines in the secrets file when re-writing
• also, keep comments and anything else untouched (except trailing whitespace in a line and trailing whitespace lines)

Also, tests for all this.