w32: Remove I/O delays due to our 100ms delay after an EAGAIN.

* src/system-w32.c (__assuan_read): Retry using select.  Map
WSAECONNRESET to EPIPE.
(__assuan_write): Retry using select.
* src/assuan-buffer.c (readline) [W32]:  Return EOF instead of EPIPE.
Release 2.1.3.

* configure.ac: Set LT version to C4/A4/R3.
Post release updates.

--
Implement socket file redirection.

* configure.ac (AC_CHECK_FUNC): Check for stat.
* src/assuan-socket.c (SUN_LEN): Add.
(eval_redirection): New.
(_assuan_sock_connect) [!W32]: Implement socket file redirection.
(_assuan_sock_set_sockaddr_un): New.
(assuan_sock_set_sockaddr_un): New.
--

This feature is useful to allow sockets with standard names even on
file system which do not supports socket and to help with shared home
directories.  For example to use GnuPG 2.1 with shared home
directories one will be abale to do:

 rm ~/.gnupg/S.gpg-agent || true
 printf "%%Assuan%%\nsocket=${HOME}/.gnupg/S.gpg-agent_${HOSTNAME}\n" \
       > ~/.gnupg/S.gpg-agent

Signed-off-by: Werner Koch <wk@gnupg.org>
Do not allow LFs in the redirected name.

* src/assuan-socket.c (eval_redirection): Stop parsing at the first
LF.
--

Avoiding LFs in file names is better for logging.