gdb/python: new styling argument to gdb.executeHEADmaster

Currently, gdb.execute emits styled output when the command is sending
its output to GDB's stdout, and produces unstyled output when the
output is going to a string.

But it is not unreasonable that a user might wish to capture styled
output from a gdb.execute call, for example, the user might want to
display the styled output are part of some larger UI output block.

At the same time, I don't think it makes sense to always produce
styled output when capturing the output in a string; if what the user
wants is to parse the output, then the style escape sequences make
this far harder.

I propose that gdb.execute gain a new argument 'styling'.  When False
we would always produce unstyled output, and when True we would
produce styled output if styling is not disabled by some other means.

For example, if GDB's 'set style enabled' is off, then I think
gdb.execute() should respect that.  My assumption here is that
gdb.execute() might be executed by some extension.  If the extension
thinks "styled output world work here", but the user hates styled
output, and has turned it off, then the extension should not be
forcing styled output on the user.

I chose 'styling' instead of 'styled' as the Python argument name
because we already use 'styling' in gdb.Value.format_string, and we
don't use 'styled' anywhere else.  This is only a little bit of
consistency, but I still think it's a good thing.

The default for 'styling' will change depending on where the output is
going.  When gdb.execute is sending the output to GDB's stdout then
the default for 'styling' is True.  When the output is going to a
string, then the default for 'styling' will be False.  Not only does
this match the existing behaviour, but I think this makes sense.  By
default we assume that output captured in a string is going to be
parsed, and therefore styling markup is unhelpful, while output going
to stdout should receive styling.

This fixes part of the problem described in PR gdb/32676.  That bug
tries to capture styled source listing in a string, which wasn't
previously possible.

There are some additional issues with capturing source code; GDB
caches the source code in the source code cache.  However, GDB doesn't
check if the cached content is styled or not.  As a consequence, if
the first time the source of a file is shown it is unstyled, then the
cached will hold the unstyled source code, and future requests will
return that unstyled source.  I'll address this issue in a separate
patch.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32676

Approved-By: Tom Tromey <tom@tromey.com>

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