Rename crc32() to crc32_compute(), to avoid clashing catastrophically

Rename crc32() to crc32_compute(), to avoid clashing catastrophically
with the crc32() function in the zlib interface. (Not that PuTTY
itself _uses_ zlib, but on Unix it's linked against libgtk which
uses libpng which uses zlib. And zlib has poor namespace management
so it defines this ridiculously intrusive function name. Arrrrgh.)

[originally from svn r3191]