add scim-im-agent man page

The man page was initially copied verbatim from the old scim-bridge-agent
package and subsequently adapted and fixed for problems.

differences to original scim-bridge man page:

- correct and improve the wording in some places
- remove whitespace at end of line
- drop and amend references to scim-bridge as necessary

Many thanks also to Tz-Huan for helping me fix a mistake I had
introduced.