add watching multiple paths at once

Now we can watch on any number of paths at once, making things a bit
easier and more efficient.

Fixes #100939
upgrade ppport.h

I tried to use av_top_index() like perlapi says I can, but it turns out
this was added in 5.17 and does not work on the 5.12 I tested it on.

ppport.h is the correct way to get new perlapi calls on older perls