treewide: make type casts explicit

[JKRhb]

As another follow-up to #126, when compiling with Visual Studio, there are a lot of warnings regarding a potential data loss due to implicit casts with a potential lossy conversion.

This PR makes the type casts in question explicit, getting rid of potential warnings.

[obgm]

Thanks for working on this. The warnings usually are there for a reason, and shutting them up with type casts is not always the best solution. One example is the return type of dtls_ccm_encrypt_message() which seems to be wrong (the result of adding two size_t values obviously should be size_t; The explicit typecast therefore should result in another warning...).
For this (and some other places), I recommend to double-check if there are no cleaner ways to address the compiler warnings.

[JKRhb]

Thank you for your feedback, @obgm! I will revisit the casts and try to come up with a cleaner solution :)

[JKRhb]

Just a quick update: I now managed to get rid of most of the type casts while keeping stricter compiler settings (adding the -Wconversion and -Wno-sign-conversion flags). However, this required changing some of the return types from int to ssize_t in order to be able to cover -1 as a return value. Is using ssize_t fine in general (since it is not a C99, but a POSIX type)? Or would a different type (such as intmax_t) be preferable here?