Clean up some function and variable names.

The change from "slave" processes to "minion" processes to "worker"
processes left some less-than-coherent names:

1. "enslave" turned into the ungrammatical "enworker".
2. "slp" (SLave Pointer) turned into "mlp" (Minion [L] Pointer?).

Convert "enworker" to "create_workers" (the function in question forks
off 3 worker processes), and replace "mlp" with "wp" (Worker Pointer)
and "tmlp" with "twp" (Temporary Worker Pointer).

Reviewed by: imp, cem, danfe
Differential Revision: https://reviews.freebsd.org/D25403