Fix instr/endin and opcode/endop detection

[kunstmusik]

Pete Goodeve reported that instr/endin and opcode/endop detection doesn't work if the keywords are indented in. Makes sense due to regex used in code.

1. Fix the detection of instr/endin and opcode/endop
2. Update the code to work even if cursor is on the lines with instr/endin/opcode/endop

[PeteCA]

This must be some weird bug in vim. I played around and found that, for instance, just doing "/^\s*endin"/ from within an instrument works just as it should, but
':let end = search("^\s*endin")' fails!
OTOH something like ':let end = search("\s*endin")' succeeds (though not useful...).

What also works is ':let end = search("^[ \t]*endin")', so I guess that would be a workaround. I've changed the plugin on my machine appropriately.