exclude: fix crashes with fortified strlcpy()

exclude: fix crashes with fortified strlcpy()

Fortified (-D_FORTIFY_SOURCE=2 for gcc) builds make strlcpy() crash when
its third parameter (size) is larger than the buffer:
  $ rsync -FFXHav '--filter=merge global-rsync-filter' Align-37-43/ xxx
  sending incremental file list
  *** buffer overflow detected ***: terminated

It's in the exclude code in setup_merge_file():
  strlcpy(y, save, MAXPATHLEN);

Note the 'y' pointer was incremented, so it no longer points to memory
with MAXPATHLEN "owned" bytes.

Fix it by remembering the number of copied bytes into the 'save' buffer
and use that instead of MAXPATHLEN which is clearly incorrect.

Fixes #511.