FUSE option -o cachesize doesn't accept exact value (without suffix)

[cipriancraciun]

If one tries to set the cache size to an exact byte value (say 268435456 which should be 256m) it complains with:

terminate called after throwing an instance of 'dwarfs::runtime_error'
  what():  invalid size suffix
Command terminated by signal 6

[mhx]

Fixed in next-release branch. The error spew is gone, too. I changed a lot in that regard for mkdwarfs but hadn't gotten to the FUSE driver yet.