uhttpd: fix crash with enabled relro, memory leak in dirlists and lfs

 * Fix the ubus plugin to not make its uhttpd_plugin entry symbol
   constant as uhttpd needs to modify its list_head member
 * Make sure that uhttpd supports large files by using 64bit ints
   where appropriate and by passing _FILE_OFFSET_BITS=64 to the build
 * Plug a possible memleak in the directory listing code

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

SVN-Revision: 44135