Split filename-based syscall implementations into a separate file

Split filename-based syscall implementations into a separate file

Create sys_filename.c, split out from nacl_syscall_common.c.  This is
for syscalls that take filenames as arguments, all of which are only
enabled in debug mode.  This naming matches the "irt_filename"
interface.

This is intended to make nacl_syscall_common.c more manageable --
currently it's quite large.

Add missing #include to nacl_host_desc.h so that the nacl_host_stat_t
type it defines is usable.

BUG=none
TEST=build

Committed: https://src.chromium.org/viewvc/native_client?view=rev&revision=12154

[bradn, 2013-09-12 21:38:15]

lgtm

[Mark Seaborn, 2013-09-12 23:01:42]

Committed patchset #2 manually as r12154 (presubmit successful).