mountd: Add check for "=" after exports(5) options

Some exports(5) options take a "=arg" component that provides an
argument value for the option. Others do not.
Without this patch, if "=arg" was provided for an option that did
not take an argument value, the "=arg" was simply ignored.
This could result in confusion w.r.t. what was being exported,
as noted by the Problem Report.

This patch adds a check for "=arg" for the options that do not
take an argument value and fails the exports line if one is found.

PR: 281003
MFC after: 2 weeks