Move the full responsibility for reporting open(2) errors from

Move the full responsibility for reporting open(2) errors from
mparse_open() to the caller.  That is better because only the caller
knows its preferred reporting method and format and only the caller
has access to all the data that should be included - like the column
number in .so processing or the current manpath in makewhatis(8).
Moving the mandoc_msg() call out is possible because the caller can
call strerror(3) just as easily as mparse_open() can.

Move mandoc_msg_setinfilename() closer to the parsing of the file
contents, to avoid problems *with* the file (like non-existence,
lack of permissions, etc.) getting misreported as problems *in*
the file.

Fix the column number reported for .so failure:
let it point to the beginning of the filename.

Taken together, this prevents makewhatis(8) from spewing confusing
messages about .so failures to stderr, a bug reported by
Raf Czlonka <rczlonka at gmail dot com> on ports@.

It also prevents mandoc(1) from issuing *two* messages for every
single .so failure.