util.signal: Wrap signalfd in an userdatum for gc handling etc

net.server_epoll: Support hooking signals via signalfd Handling signal events the same way as all other events makes sense and seems safer than the signal handling just jumping around in C and messing with Lua states.

util.signal: Add support for signalfd(2) on Linux signalfd allows handling signal events using the same method as sockets, via file descriptors. Thus all signal dispatch can go through the same main event loop as everything else, removing need for thread-scary signal handling where execution would just jump to the signal handler regardless of the state of Lua, and needing to keep track of Lua states/threads.

features: Add mod_server_info

mod_server_contact_info: Update to publish fields via new mod_server_info

mod_server_info: New module to manage the serverinfo disco extension form This allows multiple modules to populate the form dynamically. Currently the form is "owned" by mod_server_contact_info, which prevents other modules from contributing to it. A further commit will port mod_server_contact_info to use this module.

util.strbitop: Add common_prefix_bits() to Teal interface description

util.hashes: Add missing entries to Teal interface description Also sorted to match C source

util.crypto: Update Teal interface description to match C sources Was missing some entries. Rearranged to match order of entries in the C source Reg table.

tools/test_mutants.sh: Load loader helper when running busted