VTE: drop support for selecting the emulated terminal

VTE 0.38 dropped the capability to emulate various terminals to only
support xterm.  This shouldn't be so much of a problem for us, as our
setting to select the emulated terminal is hidden since 2008 and
defaults to xterm since forever, and xterm is highly compatible.
VTE: add support for vte_terminal_spawn_sync()

VTE 0.38 removed vte_terminal_fork_command() in favor of the new
vte_terminal_spawn_sync(), so add support for this.
VTE: add debugging info to known which library was actually loaded
VTE: don't hard-require vte_terminal_set_word_chars()

VTE 0.38 removed it, so only use it when available.