Whitespace fix
cksum: Optimize weechat_print callback using custom Boyer–Moore search

Instead of regexing for 'Transfer Completed', use Boyer-Moore with a
precompiled shift table. The implementation is mostly copied from
glibc's strstr(); the reimplementation allows us to save the
shift-table construction cost on every message printed.

Commit also puts all functions in cksum_* namespace, makes everything
static rather than just hidden visibility, and puts the
weechat_plugin_* strings in the read-only section.