Don't let gdb_stdlog use pager

Don't let gdb_stdlog use pager

When using the "set logging" commands, cli_interp_base::set_logging
will send gdb_stdlog output (among others) to the tee it makes for
gdb_stdout.  However, this has the side effect of also causing logging
to use the pager.  This is PR gdb/29787.

This patch fixes the problem by keeping stderr and stdlog separate
from stdout, preserving the rule that only gdb_stdout should page.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29787