Change command line option processing

[jmwilson]

There wasn't a strong consensus in #76, but it seems keeping similar command syntax is favored. This allows encrypt and decrypt options to be specified separately. The preferred syntax is now

stenc -f /dev/nst0 -e on -d mixed -k key.txt

Leaving out one of -e or -d will cause stenc to set the modes in tandem.

Breaking changes:

• -e mixed no longer works, prefer -e on -d mixed or simplify -d mixed
• Interactive prompting for keys has changed. I do not think it is practical to key in 64 hex digits manually (twice). People using the key prompt are probably cutting & pasting from a credential manager. It would be better to allow people to enter in the contents of a regular key file on standard input, so the cut & paste workflow can continue, but the key file can also be obtained by piping from a key manager. For example with Bitwarden CLI, one could do bw get <id> | stenc --encrypt=on --key-file=-
• Rename the (un)protect options as explicit --allow-raw-read or --no-allow-raw-read (following GNU style).

If approved, I will need to rewrite the man page to reflect the option syntax.

Changes command line option processing:

• Use getopt_long in libc and support long options
• GNU style option processing (don't try to enforce non-conflicting options)
• GNU style usage message
• Separate encrypt and decrypt settings
• --unprotect -> --allow-raw-read, --protect -> --no-allow-raw-read for clarification
• Change reading key file from stdin using --key-file=-
• Always print detailed status, remove summary "Drive Encryption" line

[jonasstein]

@ninthclowd do you have any objections?