Add support for multiline messages to splain

Add support for multiline messages to splain

All the ‘Compilation failed’ messages are actually multiline mes-
sages, like "Attempt to reload foo aborted.\nCompilation failed in
require at ...".

perldiag has separate entries for each line of the message, so it
makes sense to have it look up each line.  It can’t split it into
lines by default, but must check for a possible description first, as
sometimes syntax errors quote code with line breaks in it.

I had to rip out the nasty file-wide lexical $_, as I couldn’t local-
ise it, and I undid d923656e4 in the process.