Use the Linux paths for everything that isn't a Mac or Windows

Specifically, FreeBSD. Assuming something POSIX-like if there is no
specific code for the target OS is a good idea in general, though.
Correct a comment about GetDataString safety
Remove unused MSVC project files for tstc4netio and netpuncher
Update dependencies list in the README

Also remove mention of PATH from windows installer section - CMake finds
makensis using the Registry when it isn't in the PATH.