allow setting of 'max_pages' (via 'fuse_msg_size')

Linux 4.20 and above allow setting the number of pages per FUSE message
upto 256 (4K * 256 = 1MiB). This can greatly increase read and write
speeds depending on the workload.

src/config.cpp

@@ -53,6 +53,7 @@ Config::Config()
     cache_readdir(false),
     async_read(true),
     cache_files(CacheFiles::LIBFUSE),
+    fuse_msg_size(FUSE_MAX_MAX_PAGES),
     POLICYINIT(access),
     POLICYINIT(chmod),
     POLICYINIT(chown),