Change output_write() to writebuf(), add readbuf() helper.

Change output_write() to writebuf(), add readbuf() helper.

* src/misc.c (writebuf, readbuf): Create helper functions that will
reliably write a buffer to a file descriptor in the face of EINTR
causing short writes, and read from a file descriptor into a buffer
in the face of EINTR causing short reads.
* src/makeint.h: Declare these functions.
* src/output.c: Remove output_write() and replace with writebuf().
(_outputs, out_of_memory): Call writebuf(), not output_write().
* src/arscan.c (ar_scan): Call readbuf() instead of read(2).
(ar_member_touch): Remove duplicate header write, call writebuf()
instead of output_write(), and readbuf() instead of read(2).