Only lines consisting of nothing but dashes are body separators

Only lines consisting of nothing but dashes are body separators

This is how the documentation specifies it, see mh-mail(5). The old
m_getfld(), however, was more relaxed and treated any header line that
starts with a dash as the body separator. We are more strict here,
because ``-foo: bar'' is a valid header line (according to RFC 2822),
that should thus not be treated as the body separator, whereas a line
``------'' in the header could only be a format error if we don't
take it as the body separator.