First cleanup pass on sysctl.

Remove unnecessary typecasting of things that are already char * to char *,
and multiplying by sizeof(char *) which is 1. Rename do_process() do_flag_a()
since that's the only caller. Move read_key_values() down past trim_spaces()
and read_config_file() so it's next to its only two users. Replace some
euphemisms for 0 with 0.

replace_char() really sounds like something libc should already have one of,
but I'm blanking on it if so. (It doesn't need a temporary variable when the
argument variable is already a copy so changing it won't affect the caller.)
Cleanup pass on sysctl.

Not heavily tested yet but should be finished at the design level.