Change neo* configurations to n* configurations
Extract pos.h from structs.h

This will make it much simpler to move the other types in structs.h
which depend on `pos_T` and `lpos_T`.
Move exarg_T and cmdarg_T from structs.h to normal.h
refactored logic from init_users() into mch_get_usernames()
moved mch_get_user_name() and mch_get_uname() into os/users.c
removed get_user_name() and replaced calls with mch_get_user_name()
refactored part of expand_env_esc() into mch_get_user_directory()
fix segfault on linux
fixed code style issues