Problem: Command line history code is spread out.
Solution: Put the code in a new file. (Yegappan Lakshmanan, closes#4779)
Also graduate the +cmdline_hist feature.
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: Ex command info contains confusing information.
Solution: When using the NOTADR flag use ADDR_OTHER for the address type.
Cleanup code using NOTADR. Check for errors in
create_cmdidxs.vim. Adjust Makefile to see the errors.
Problem: Support for user commands is spread out. No good reason to make
user commands optional.
Solution: Move user command support to usercmd.c. Always enable the
user_commands feature.
