warnings: fix strncpy warnings

GCC is getting smarter and detects that we're not copying the NUL terminator over.

There are two approaches to fixing the warning:

• where code is already a bit more complicated, just use std::string instead of C-style string manipulations; and
• where we were doing strncpy(dest, src, strlen(src)), just use strcpy instead since the destination sizes are made based on the size of src anyways.

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This fixes warnings which have appeared on megas due to its Fedora 28 update.

Cc: @utkarsh.ayachit @cory.quammen @brad.king @ken-martin