Problem:
1. `vim_getenv` is followed by `TO_SLASH` when getting
path-related variables.
2. cmd exits when launched with forward slash.
Solution:
1. try calling `TO_SLASH` in `vim_getenv`.
2. pass fullpath via `lpApplicationName`, only include `cmd.exe`
in cmdline.
Co-authored-by: Justin M. Keyes <justinkz@gmail.com>