add symbol hover support for codesearch.debian.net

When we are showing symbol info for a macro or a function
in a c file, then add a link to search the source code
of that macro/function in codesearch.debian.net .

It uses the codesearch.debian.net regex search with the
following regex which looks up source definitions for C
macros and functions:

(?mi:^(?:#define\s+)?func_name)\s*\((?m:([^;]+)$)
3 jobs for 0-codesearch-debian-net in 35 minutes and 48 seconds
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