Rename config key displayname to maildisplayname, and refactor. (#33)

Rename config key displayname to maildisplayname, and refactor. (#33)

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)

Full commit list:
* fix compilation error (couldn't find selinux.h)
* Rename boolean variable names.
* Check for stdbool.h's availability.
* Refactor maildisplayname.
* Move format_displayname() and make_mailbox_addr() into their own file.
* Ignore test binary.
* Rename option displayname to maildisplayname.