Import OpenBSD's bcrypt implementation to use bcrypt_pbkdf().

Also, rename the Linux Makefile to represent the purpose of it. It is
still a GNU make file, but it only used on Linux, so... I don't know,
really...

Various assortments in the Linux Makefile, and extend it, to compile the
newly added bcrypt/ directory as a dependency to kc on Linux.

The bcrypt_pbkdf() KDF is not used yet in this commit, however it is
coming.
Hook up the bcrypt KDF, and update the manual and help accordingly.

Also, clean up the manual's CIPHERS and KDF sections. Specifically a
major fuck-up on my part in the CIPHERS section, regarding not knowing
what the hell is the difference between bits and bytes... well, I can
take it, that's not the worst misunderstanding that had happened to
me... :P