Add new parameter --lock-suffix. (#287)

Add new parameter --lock-suffix. (#287)

* Add new parameter --lock-suffix.

This enables automation systems to run multiple instances of dehydrated
while still maintaining a locking facility. This is necessary for
projects like https://github.com/GUI/lua-resty-auto-ssl.

With this feature, one could run the script multiple times, while still
having a form of locking:

./dehydrated --lock-suffix test1.example.com -d test1.example.com
./dehydrated --lock-suffix test2.example.com -d test2.example.com
./dehydrated --lock-suffix test3.example.com -d test3.example.com
./dehydrated --lock-suffix test4.example.com -d test4.example.com
./dehydrated --lock-suffix test5.example.com -d test5.example.com

When starts the script with the same locking suffix, the scripts exits
like it normally would when locked. This will give you the benefits of
using --no-lock, without the disadvantages of it.

* Fixed unbound variable error for new PARAM_LOCKFILE_SUFFIX.