Fix Savannah bug 40241 with Unixy file names as commands to MSYS shell.

* sub_proc.c: Include filedef.h and variable.h.
(process_begin): If exec_path was not found, but its first
character is '/', assume there's some shell magic, and invoke the
command through '$(SHELL) -c "COMMAND"'.  Fixes SV bug#40241.
(make_command_line): Kludgey feature: if full_exec_path is "-c",
assume that argv[0] is not to be skipped, as it holds the command
string to be passed to the shell.
Fix Savannah bug #31150 with failures due to setting window title.

* sub_proc.c (process_begin): Don't set startInfo.lpTitle, it
reportedly causes SV bug #31150, and according to MSDN it's a
no-no.