(gdb-source-info): Don't display source at startup, if required.
(gdb-show-main): New option.
(gdba): Update documentation.
(gdb-source): Cover case of auto-display output.
(gud-gdba-marker-filter): Log the process input and output, if
required. (from Stefan Monnier)
(gdb-debug-log, gdb-enable-debug-log): New variables.
(gdb-post-prompt): Don't do gdb-var-update on Mac OS X.