Add --passphrase-fd option.

[sz-coder]

This is my first attempt to get age reading a passphrase from a file descriptor.

See discussion.

[sz-coder]

Of course there are still some caveats that need to be addressed:

• New line character (0xA) should be trimmed when reading from a file descriptor. (I think?)
• Emit a warning if the passphrase does not fit completely in the buffer (currently 1024 bytes).
• Move logic out of passphrasePromptForEncryption function.
• Compatibility with other operating systems such as windows (I don't know if windows has the concepts of file descriptors).

[FiloSottile]

#275 (comment)

[clach04]

...

• Compatibility with other operating systems such as windows (I don't know if windows has the concepts of file descriptors).

This would be a non-starter on Windows (but I can see the use for non-Windows).

Cross-referencing related/duplicated discussion/PR topics:

• UX: Make age take a passphrase from a non-terminal source #275 "UX: Make age take a passphrase from a non-terminal source"
  • Add --passphrase-fd option. #276 "Add --passphrase-fd option."
• Read passphrase from AGE_PASSPHRASE env var if available #346 "Read passphrase from AGE_PASSPHRASE env var if available"
• cmd/age: passphrase optionnaly supplied from an environment variable #520 "cmd/age: passphrase optionnaly supplied from an environment variable"