VFS: add BSD socket API, socket driver support

This patch adds the implementation of the BSD socket system calls
which have been introduced in an earlier patch. At the same time, it
adds support fo

VFS: add BSD socket API, socket driver support

This patch adds the implementation of the BSD socket system calls
which have been introduced in an earlier patch. At the same time, it
adds support for communication with socket drivers, using a new
"socket device" (SDEV_) protocol. These two parts, implemented in
socket.c and sdev.c respectively, form the upper and lower halves of
the new BSD socket support in VFS. New mapping functionality for
socket domains and drivers is added as well, implemented in smap.c.

The rest of the changes mainly facilitate the separation of character
and socket driver calls, and do not make any fundamental alterations.
For example, while this patch changes VFS's select.c rather heavily,
the new select logic for socket drivers is the exact same as for
character drivers; the changes mainly separate the driver type
specific parts from the generic select logic further than before.

Change-Id: I2f13084dd3c8d3a68bfc69da0621120c8291f707

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