Add kstack decoder

[bobrik]

From the example:

ivan@vm:~/projects/ebpf_exporter$ curl -s http://ip6-localhost:9435/metrics | fgrep accessed
ebpf_exporter_ebpf_program_info{config="kstack",id="1803",program="mark_page_accessed",tag="4fe6d77dbc5615bc"} 1
# HELP ebpf_exporter_mark_page_accessed_total Total number of calls to mark_page_accessed by kstack
# TYPE ebpf_exporter_mark_page_accessed_total counter
ebpf_exporter_mark_page_accessed_total{kstack="mark_page_accessed\nfollow_page_pte\nfollow_page_mask\n__get_user_pages\nget_user_pages_remote\nget_arg_page\ncopy_string_kernel\ndo_execveat_common\n__arm64_sys_execve\ninvoke_syscall.constprop.0\ndo_el0_svc\nel0_svc\nel0t_64_sync_handler\nel0t_64_sync"} 5
ebpf_exporter_mark_page_accessed_total{kstack="mark_page_accessed\nfollow_page_pte\nfollow_page_mask\n__get_user_pages\nget_user_pages_remote\nget_arg_page\ncopy_strings\ndo_execveat_common\n__arm64_sys_execve\ninvoke_syscall.constprop.0\ndo_el0_svc\nel0_svc\nel0t_64_sync_handler\nel0t_64_sync"} 10
ebpf_exporter_mark_page_accessed_total{kstack="mark_page_accessed\nunmap_page_range\nunmap_vmas\nexit_mmap\n__mmput\nmmput\nbegin_new_exec\nload_elf_binary\nbprm_execve\ndo_execveat_common\n__arm64_sys_execve\ninvoke_syscall.constprop.0\ndo_el0_svc\nel0_svc\nel0t_64_sync_handler\nel0t_64_sync"} 914
ebpf_exporter_mark_page_accessed_total{kstack="mark_page_accessed\nunmap_page_range\nunmap_vmas\nunmap_region.constprop.0\ndo_vmi_align_munmap\ndo_vmi_munmap\n__vm_munmap\n__arm64_sys_munmap\ninvoke_syscall.constprop.0\ndo_el0_svc\nel0_svc\nel0t_64_sync_handler\nel0t_64_sync"} 15

This isn't as useful in metrics, but it would be more natural in tracing.