Problem: Alloc() returning "char_u *" causes a lot of type casts.
Solution: Have it return "void *". (Mike Williams) Define ALLOC_ONE() to
check the simple allocations.
Problem: Using "int" for alloc() often results in compiler warnings.
Solution: Use "size_t" and remove type casts. Remove alloc_check(), Vim
only works with 32 bit ints anyway.
Problem: The do_tag() function is too long.
Solution: Factor parts out to separate functions. Move simplify_filename()
to a file where it fits better. (Andy Massimino, closes#4195)
