Fix bug where emufs mappings aren't coalesced properly
Add AddressSpace::for_all_mappings
Add v4l2 structs to kernel_abi
Avoid sign-extension in HEX helper
Fix comment in Task::fstat
Add TaskSyscallState::after_syscall_action to run arbitrary code when we finish recording a syscall exit
Handle cases where it's possible to mmap portions of a zero-sized device file
Implement support for V4L2 ioctls

Resolves #1408.
Replace record_page_below_stack_ptr flag with after_syscall_action mechanism
Fix quotactl handling, and add a test