add --prefer-only option to only kill processes matching --prefer

[chombourger]

While --avoid exists to list processes that should not be killed, it may be desirable to have earlyoom simply ignore any processes that do not match the regex specified in --prefer. It should be noted that the Linux kernel may still opt to kill other processes if earlyoom did not free up enough memory to prevent an oom.

[rfjakob]

Looks good, please rename the flag to --only.