lib/ukalloc: Introduce `uk_alloc_maxalloc()` and make `uk_alloc_availmem()` available

lib/ukalloc: Introduce `uk_alloc_maxalloc()` and make `uk_alloc_availmem()` available

The purpose of this commit is to make `uk_alloc_availmem()` always
available because it is currently the only way to figure out how much heap
memory is still available. We also introduce `uk_alloc_maxalloc()` that
returns the biggest possible allocation that an allocator can currently
satisfy. The configuration option `CONFIG_IFSTAT` is removed because it
raises the expectation of an interface that returns detailed allocator
statistics.

Signed-off-by: Simon Kuenzer <simon.kuenzer@neclab.eu>
Reviewed-by: Cezar Craciunoiu <cezar.craciunoiu@gmail.com>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Pull-Request: #229