Fix handling of non-integer START param to write-region

Fix handling of non-integer START param to write-region

The previous patch for Bug#354 incorrectly assumed that START would
always be an integer.

* lisp/epa-file.el (epa-file-write-region):
* lisp/jka-compr.el (jka-compr-write-region):
* lisp/net/ange-ftp.el (ange-ftp-write-region):
* lisp/net/tramp-gvfs.el (tramp-gvfs-handle-write-region):
* lisp/net/tramp-sh.el (tramp-sh-handle-write-region):
* src/fileio.c (write_region): Handle nil and string values of START.