* src/pty.c(_vte_pty_run_on_pty): exit on any error, to avoid leaving random
	unreaped children running.  Accept NULL command indicating that no
	exec() should occur, but still error out if command isn't NULL and
	exec() fails.
* src/pty.c(_vte_pty_fork_on_pty_name,_vte_pty_fork_on_pty_fd): store 0 as the
	new child's PID if we're the child.
* src/pty.c(main): don't run "tty" by default, test the forkpty() case instead.
* src/vte.c(vte_terminal_fork_command): rename to _vte_terminal_fork_basic.
	Remove code to replace NULL command with the user's shell.
* src/vte.c(vte_terminal_fork_command): new (sort of)! wrap
	_vte_terminal_fork_basic, replacing a NULL command with the user's
	shell.
* src/vte.c(vte_terminal_forkpty),src/vte.h: new! wrap _vte_terminal_fork_basic,
	passing NULL for the command and argv arguments (bug #116450).
* src/Makefile.am: update shared library version.
* doc/reference/vte-sections.txt: add vte_terminal_forkpty.