system-name's returned value can vary

Also, the system-name variable is now obsolete.
Fixes Bug#19438.
* doc/lispref/os.texi (System Environment):
* etc/NEWS: Document this.
* doc/misc/efaq.texi:
(Displaying the current file name in the titlebar):
* lisp/desktop.el (desktop-save-frameset):
* lisp/dnd.el (dnd-get-local-file-uri):
* lisp/gnus/message.el (message-make-fqdn):
* lisp/gnus/nnvirtual.el (nnvirtual-retrieve-headers)
(nnvirtual-update-xref-header):
* lisp/nxml/rng-uri.el (rng-uri-file-name-1):
* lisp/org/org-clock.el (org-clock-save):
* src/filelock.c (current_lock_owner):
* src/xrdb.c (get_environ_db):
* src/xterm.c (same_x_server):
* src/xterm.c (x_term_init):
Prefer (system-name) to system-name, and avoid naming
locals 'system-name'.
* doc/misc/smtpmail.texi (Server workarounds): Fix grammar.
* lisp/startup.el (system-name): Now an obsolete variable.
* src/editfns.c (cached_system_name): New static var.
(init_and_cache_system_name): New function.
(init_editfns, Fsystem_name): Use it.
(syms_of_editfns): Initialize it and Vsystem_name to the same value.
* src/sysdep.c [HAVE_SOCKETS]: Don't include <sys/socket.h>, <netdb.h>.
(h_errno) [TRY_AGAIN && !HAVE_H_ERRNO]: Remove decl.
(init_system_name) [HAVE_SOCKETS]: Don't canonicalize the name.
Don't create a new string if the current value is already correct.
Fix previous patch to match its commit message
* obsolete/pc-select.el (pc-selection-mode): Use system-type.

This is instead of system-name, which is both wrong here and obsolete.
Fix the MS-Windows build.

 src/sysdep.c [WINDOWSNT]: Include sys/socket.h, without which this
 file doesn't compile on MS-Windows.