Add helper tool to generate the key file

[sunwire]

A simple tool to generate a key for stenc. It uses openssl command to do that.

Usage: stenc-keygen.sh -l 16 -d "key descriptor" -f "key filename.key" Where:

• -l key length 16 or 32 bytes
• -d key descriptor
• -f filename

All options are optional. By default key length is 32 bytes, key descriptor is TK + current date (YMD) and time (HMS) (digits only), filename is key descriptor + .key extension.

E.g.

$ stenc-keygen.sh 
Generating the key...
Key filename: TK220616151224.key

$ stenc-keygen.sh -d "June key"
Generating the key...
Key filename: June key.key

$ stenc-keygen.sh -d "June key" -f June.key
Generating the key...
Key filename: June.key

[jonasstein]

however we can improve the man page according to key generation, if you have ideas...