Fix adb forward --list when forwarding a lot

The list action had some problems with large numbers of forwards:
 * adb_query() limited replies to 1024 B (and the print was useless)
 * the reply header's length could overflow (also in other commands)
 * ...and the client had no way of detecting it
 * writex() didn't retry on EAGAIN ("Resource temporarily unavailable")

This patch makes all "OKAY%04x" replies use a common function which
checks the length and limits it to 0xffff. This means that the client
can easily check for truncated replies.

Before: forward --list starts failing at 15-30 forwards (depending on
device serial and forward spec lengths).

After: no problems with forward --list.

Change-Id: Ie1e82c4d622f5c56e51abb26533ba17d40459914