kill: support signal number 0

Tests process existence, but does not send a signal. Matches POSIX
behavior.

Fix delivery of signals when a signal uses the default handler, make context status update on every switch

Fix memory leaks in exec
Remove warnings

Use slab allocator for kernel heap

Operate on word size as possible

Fix #79 by limiting mappable sections to the 2GB mark

Update debugging docs

Write hpet timer twice

This allows booting with implementations that require them to be set separately.
Also, check for the availability of legacy-replacement mode and periodic interrupts before using hpet

Fix issues with SIGCONT