bfd: Implement buffered writes

Dump times (top-5) look like

Before patch:
	writev:     1595  0.048337 (15.1%)
	openat:     1326  0.041976 (13.1%)
	 close:     1434  0.034661 (10.8%)
	  read:      988  0.028760 (9.0%)
	 wait4:      170  0.028271 (8.8%)

After patch:
	openat:     1326  0.040010 (16.4%)
	 close:     1434  0.030039 (12.3%)
	  read:      988  0.025827 (10.6%)
	 wait4:      170  0.025549 (10.5%)
	ptrace:      834  0.021624 (8.9%)

So write-s go away from top list (turn into 8th position).

Funny thing is that all object writes get merged with the
magic writes, so the total amount of write()-s (not writev-s)
in the strace remain intact :)

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>