Rename config key displayname to maildisplayname, and refactor.

[yo8192]

The main changes are:

• rename the fcron.conf config key displayname into maildisplayname, for clarity.
• move the underlying code into its own mail.c file, with some refactoring to improve clarity and simplify slightly.
• extend the extra overflow tests to test at the limit, and just over the limit (expecting an overflow error)
• ensure things work on systems which don't have stdbool.h (which is C99)