Make KEYBD trap handling code multibyte-aware

[McDutchie]

For background, see #307 and 6b77da2.

To do:

1. Figure out how the edit.c code works
2. Remove workaround applied in 6b77da2
3. Rewrite what needs to be rewritten to make keytrap() and the code that calls it in ed_getchar() multibyte aware

[hckiang]

If you have figured out how edit.c works, it may also be good to extend the buffer size of .sh.edchar by the way. Currently if you set .sh.edchar to a longer string (say more than 100 chararacters) in the KEYBD trap the sequence will just be truncated. I have my own programmable autocompletion using the .sh.edchar mechanism but my completion is sometimes truncated because of this limitation.

[posguy99]

I have my own programmable autocompletion...

Is that something you can share? Sounds very useful.

[hckiang]

very useful

I use it myself daily but it maybe not so useful for others yet because it's kind of hacky and half-baked. Nonetheless I've just uploaded it here. See the comments in autocmpt.sh to see how to use.