Pull up following revision(s) (requested by gdt in ticket #1534):

sys/kern/vfs_syscalls.c: revision 1.544
lib/libc/sys/fdatasync.2: revision 1.17

Relax fdatasync restriction that fd be writable

The restriction that a fd passed to fdatasync(2) must be writable was
added in 2003 in order to comply with POSIX.  Since then, POSIX has
removed that requirement, and POSIX-valid programs have been therefore
encountering errors on NetBSD.

Patch by Paul Ripke after discussion on netbsd-users.  Issue
discovered with pkgsrc/databases/mongodb3 as used by pkgsrc/net/unifi.

(martin)