MAC/do: parse_rules(): Tolerate blanks around tokens

To this end, we introduce the strsep_noblanks() function, designed to be
a drop-in replacement for strstep(), and use it in place of the latter.

We had taken care of calling strsep() even when the remaining sub-string
was not delimited (i.e., with empty string as its second argument), so
this commit only has mechanical replacements of existing calls.

Reviewed by: bapt
Approved by: markj (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47623