Use POSIX-specified <poll.h> over <sys/poll.h>

POSIX specifies that <poll.h> is the correct header to include for
poll(), so let's do that instead.

https://bugzilla.gnome.org/show_bug.cgi?id=141251
GVariant: add way to print context of parse errors

This was a feature intended from the very beginning that somehow never
got written.  It's a way to replace these sort of error messages out of
the GVariant parser:

  1-2,10-15:unable to find a common type

with something in the style of the Vala compiler:

  unable to find a common type:
    [1, 2, 3, 'str']
     ^        ^^^^^

https://bugzilla.gnome.org/show_bug.cgi?id=715028
gsettings tool: print GVariant errors in context

Use g_variant_parse_error_print_context() to format the error message
from the GVariant parser.

https://bugzilla.gnome.org/show_bug.cgi?id=715028
gapplication(1): print GVariant errors in context

Use g_variant_parse_error_print_context() to format the error message
from the GVariant parser.

https://bugzilla.gnome.org/show_bug.cgi?id=715028
gdbus tool: print GVariant errors in context

Use g_variant_parse_error_print_context() to format the error message
from the GVariant parser.

There is a slightly dubious interaction with the "parse me harder"
functionality here.  We're probably going to have to deal with that
separately.

https://bugzilla.gnome.org/show_bug.cgi?id=715028