rc: Disable pathname expansion when calling run_rc_command()

Variables for command-line options like $foo_flags can contain characters
that perform pathname expansions, such as '[', ']', and '*'. They were
passed without escaping, and the matched entries in the working directory
affected the command-line options. This change turns off the expansion
when run_rc_command() is called.

While this changes the current behavior, an invocation of a service
program should not depend on entries in the working directory.

Differential Revision: https://reviews.freebsd.org/D45855