Fix and improve GUD Tooltip mode.

Fix and improve GUD Tooltip mode.

 lisp/tooltip.el (tooltip-identifier-from-point): Don't treat tokens
 inside comments and strings as identifiers.
 lisp/progmodes/gud.el (gud-tooltip-print-command): Quote the
 expression to evaluate.  This allows to evaluate expressions with
 embedded whitespace.
 (gud-tooltip-tips): Add a blank before the newline in the
 message-box text, for the benefit of message-box emulation on
 MS-Windows.
 lisp/progmodes/gdb-mi.el (gdb-tooltip-print): Don't ignore error
 messages from GDB, pop them up in a tooltip to give feedback to
 user.
 (gdb-tooltip-print-1): Quote the expression to evaluate.  This
 allows to evaluate expressions with embedded whitespace.
 (gdb-inferior-io--init-proc): Don't send "-inferior-tty" command
 if the TTY name is nil or empty (which happens when communicating
 with the inferior via pipes, e.g. on MS-Windows).
 (gdb-internals): If GDB sends a "&\n" empty debugging message,
 don't send that to the GUD buffer.

 doc/emacs/building.texi (Debugger Operation): Correct and improve
 documentation of the GUD Tooltip mode.