mhmail: For mailx(1) compat: A period alone on a line means EOF.

Also renamed the -body switch to -bodytext because its argument is the
body text and not a file containing the body text. The rename should
clarify this. After all, typing `-body' results in the same as typing
`-bodytext' or only `-b'.
Now, fgets() is used instead of fread(), thus not allowing null-bytes
in the input anymore. This should be no restriction.