cleaned up foreign.c

need to actually interpret the filename, oops

$ vips copy wtc.jpg x.jpg[Q=100]
$ ls -l x.jpg\[Q\=100\]
-rw-rw-r-- 1 john john 15133033 Apr 25 14:50 x.jpg[Q=100]
$ vips copy wtc.jpg x.jpg[Q=1]
$ ls -l x.jpg\[Q\=1\]
-rw-rw-r-- 1 john john 15133033 Apr 25 14:50 x.jpg[Q=1]
fix up filename options

all done in vips_foreign_load() / vips_foreign_save() now

added vips__filename_split8()