Split manual pages into per-action page and use AsciiDoc format

Use pre-generated man pages in make dist.

[Added fixes and updates from Ondrej Kozina and Milan Broz]

man/cryptsetup-luksAddKey.8.adoc

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+= cryptsetup-luksAddKey(8)
+:doctype: manpage
+:manmanual: Maintenance Commands
+:mansource: cryptsetup {release-version}
+:man-linkstyle: pass:[blue R < >]
+:COMMON_OPTIONS:
+:ACTION_LUKSADDKEY:
+
+== Name
+
+cryptsetup-luksAddKey - add a new passphrase
+
+== SYNOPSIS
+
+*cryptsetup _luksAddKey_ [<options>] <device> [<key file with new key>]*
+
+== DESCRIPTION
+
+Adds a new passphrase. An existing passphrase must be supplied
+interactively or via --key-file. The new passphrase to be added can be
+specified interactively or read from the file given as the positional
+argument.
+
+*NOTE:* with --unbound option the action creates new unbound LUKS2
+keyslot. The keyslot cannot be used for device activation. If you don't
+pass new key via --volume-key-file option, new random key is generated.
+Existing passphrase for any active keyslot is not required.
+
+*NOTE:* some parameters are effective only if used with LUKS2 format
+that supports per-keyslot parameters. For LUKS1, PBKDF type and hash
+algorithm is always the same for all keyslots.
+
+*<options>* can be [--key-file, --keyfile-offset, --keyfile-size,
+--new-keyfile-offset, --new-keyfile-size, --key-slot, --volume-key-file,
+--force-password, --hash, --header, --disable-locks, --iter-time,
+--pbkdf, --pbkdf-force-iterations, --pbkdf-memory, --pbkdf-parallel,
+--unbound, --type, --keyslot-cipher, --keyslot-key-size].
+
+include::man/common_options.adoc[]
+include::man/common_footer.adoc[]