Correct default lines of scrollback in the docs.

The default was increased in fd266a3 (just before 0.63).
Refer to X11 'graphical applications'.

For people who know they need graphical applications but don't know what
X11 is.
Clarify that port forwarding is of TCP connections.

(Rather than just the generic 'network connections'.)
Rearrange SSH bug docs to match the GUI.

No change to the text.
Tweak SSH protocol version refusal messages.

"required by user" will grate if the user did not configure the
behaviour (and I'm about to change the default to `2 only').
Tweak key file config docs re Pageant.

Clarify that it's not necessary to faff around with explicit key
configuration if you're using Pageant.
Don't try SSH-1 by default.
Cross-reference 'logical host' and host key config docs.
Discourage SSH-1 in documentation.
Merge misc docs improvements from branch 'pre-0.64'.
Merge SSH-2-only default from 'pre-0.64'.
Repair GTK timer handling

The bug eventually makes PuTTY take 100% of a CPU.

timing.c:  This patch stops run_timers returning with a next timer
interval of zero ms. Which gets rid of the CPU spinning caused by other
parts of the code needing the timer to have passed.

unix/gtkwin.c: The code called from run_timers is likely to create a
new timer when it runs before timer_trigger adds the next timer that
run_timers returns. This would mean that two timers get scheduled
with gtk.

terminal.c: Make sure terminal.c doesn't waste CPU doing the text blink
timer when we don't have focus. Naturally the text blink and a blinking
cursor will resync when we regain focus.

NB: Windows calls 'term_set_focus' a lot .. Yes you still have focus
Repair GTK timer handling: Partial revert.

Something similar is now in SGT's base code.
Pango font performance update

This change improves the speed of the best case pango font
processing.  It removes multiple calls to pango_layout_set_text and
pango_layout_get_pixel_extents that were used to check that every prefix
string of a given draw_text is actually the correct size.

Instead the function now checks just once before painting the text
that the size of the whole region is correct. If it's not correct it
falls back to painting one character at a time immediatly rather than
optimistically trying lots of substrings; this normally improves the
draw time of complex screens and proportional fonts too.

If there are compensating layout issues within the string they won't
cause problems because terminal.c already has code to redraw any row
of text that was drawn as one operation if it's split.
Minimise, maximise, etc are as annoying as other terminal resize operations.

Note they ARE resize operations so treat them as such and disable them
if requested.
Add XTerm extensions for VTTEST and reverse BS

Codes to enable and disable the DECCOLM 132/80 column switching DEC
control and XTerm's reverse BS hack to 'pass' the vttest option 1.

This means that PuTTY uses the same modal hack as XTerm to pass the vttest
'end of line' test, (Actually the reverse part) and Thomas can stop lying
about PuTTY in the xterm FAQ.  The second part prevents the default xterm
'init' sequence from terminfo clearing the screen.

In addition most applications realise that terminal resizes are requests
and that you likely won't get what you ask for.  Programs (like vttest)
that use the sequence DECCOLM aren't so smart so the change is immediatly
enacted.
Adjust DEC-MCS character set.

U+2426 "␦" is defined as the substitution character in DEC-MCS

Note: dec_mcs is a 94 character set so 0xA0 and 0xFF are not defined in
a different way when it's mapped into GR.
Optimisation to delete windows code on unix

Changed is_dbcs_leadbyte to a macro on unix so the compiler knows
it's always zero.
Windows codepages do not have C1 controls

MS CP125X do NOT actually have any high control characters, the MS tables
show some of them but they are never interpreted by Microsoft software
and, as they have done before, MS will replace them by any new characters
that may be useful in the future.

Also eliminates as possible security risk where control characters are
not expected in Windows csets by host software.
Fix the scoasc translation on unix

There should be a set of VGA ROM characters in the first 32 positions
of the character set to match with Linux, scoansi and Windows PuTTY.