Fix build on 8-bit platforms (like Arduino!)

Add buildability on OpenBSD and NetBSD.

Add note about GPG-signing my commits.

Clarify the 'si' parameter of s20_crypt

Remove generated C code from unit_tests.c

Switch literal escape character to \033 in aunit.h

Derp. Did not completely switch out ERROR for FAILURE (see 87eb4b7a714). Thanks @haffs!

Add sanity check that buf isn't NULL

Added tests for s20_crypt argument sanity checks

Check for correct enum, when s20_returns

Bring comment up to date :-(

Switch ERROR for FAILURE, as it's meant to be the opposite of SUCCESS

Fix spelling mistake

Rename 'len' parameter to 'buflen' for consistency with 'key' and 'keylen'

Replace 'int' return type of s20_crypt with an enum