Also add "make syntax-check" rules to ensure no new uses sneak in.

There are many uses of write like this:

    if (write (fd, xml, towrite) != towrite)
        return -1;

The problem is that the syscall can succeed, yet write less than
the requested number of bytes, so the caller should retry
rather than simply failing.

This patch changes most of them to use util.c's safewrite wrapper,
which encapsulates the process.  Also, there were a few cases in
which the retry loop was open-coded, and I replaced those, too.

* Makefile.maint (sc_avoid_write): New rule, to avoid recurrence.
* .x-sc_avoid_write: New file.  Record two legitimate exemptions.
* qemud/qemud.c (sig_handler, qemudClientWriteBuf): Use safewrite, not write.
* src/conf.c (__virConfWriteFile): Likewise.
* src/qemu_conf.c (qemudSaveConfig, qemudSaveNetworkConfig): Likewise.
* src/qemu_driver.c (qemudWaitForMonitor, qemudStartVMDaemon)
(qemudVMData, PROC_IP_FORWARD): Likewise.
* proxy/libvirt_proxy.c: Include "util.h".
(proxyWriteClientSocket): Use safewrite.
* src/test.c (testDomainSave, testDomainCoreDump): Likewise.
* src/proxy_internal.c (virProxyWriteClientSocket): Likewise.
* src/virsh.c: Include "util-lib.h".
(vshOutputLogFile): Use safewrite.
* src/console.c: Include "util-lib.h".
(vshRunConsole): Use safewrite.