ensure qw//, q'' and '' report line on a missing terminator

scan_str() only sets PL_multi_end on success, but the qw, q amd '' cases
were calling the COPLINE_SET_FROM_MULTI_END; macro before reporting
failure, overwriting the line used for reporting errors.

For the simplest case, as in the ticket and the tests, PL_multi_end is
zero, so the error is reported without a line number.
perlapi: Give more accurate value for needed free space

When converting to UTF-8, one usually doesn't need 14 bytes available
space, which is what previously was claimed  It acutally depends on the
value being converted.  This change gives the precise value.
perlsub: Fix too-long verbatim line length

This was introduced by commit d3f8a934ef964c0f488e9c692275435d8ea2e291
which reverted earlier fixes, including this pod problem.