Core/Spells: Fix a warning possibly introduced in da5d0da7
Core/Packets: Added SMSG_TRAINER_LIST and fixed build
DB/Schema/Auth: Also change `battlenet_accounts`.`os` to varchar(4)

Ref 59bb9c20f345544310369f3074fd0e716cc8092c
Core: Use the correct function to convert strings to unsigned longs and unsigned long longs

Fixes wrong data being inserted into the database (i.e explored zones).
More info in #13493

Fixes #13493

Thanks to @jackpoz for finding the code issue and @Vavehl for an expectional bug report.
README: Update 3.3.5 build status badge
Core/Packets: Fixed Item.db2 and Item-sparse.db2 query replies
Core/Packets: Updated some of the movement flag opcodes