Document all gdbus-codegen options

The --help and --xml-files options were missing from the man page.
Fix a typo
Document all glib-genmarshal options

The --stdinc option was missing from the man page.
Document all glib-mkenums placeholders

The @ENUMPREFIX@ placeholder was missing in the man page.
Document all gtester-report options

The --subunit option was missing from the man page.
Remove a preprocessed-out code section

If it has not been needed since 2000, there probably is no use for it.
Started factoring the poll backend out of GMainContext
Factored out the poll-centric API support functions

The methods of GMainContext that assume it is backed by a GPollFunc
will be supported only by the poll backend. The context will switch
to the poll backend to complete any of these functions.
Provide a user data pointer to custom main contexts

It could be used to pass a uv_loop_t* for the libuv backend,
for example.
Added acquire and release functions for main loop backends