use BPF_HISTOGRAM and num_open_kprobes

Helper to get open k[ret]probes. Fixes #236

Add BPF_HISTOGRAM type and print support

Signed-off-by: Brenden Blanco <bblanco@plumgrid.com>

This adds support for a specialized histogram type, which underneath
maps to an array or a hash table, depending on key type. With no
arguments, it takes on the type `u64 table[64];`. The other current
supported key type is `struct { int32|int64 bucket; int32|int64 slot }`.

To print these automatically, print_log2_hist is underneath split into
two types of printouts, one which prints the single histogram, and
another which prints a histogram for each unique `bucket` value.

See test_histogram.py for examples.

Fixes: #144
Signed-off-by: Brenden Blanco <bblanco@plumgrid.com>

Allows things like:
```c
int kprobe__foo(struct pt_regs *ctx, u64 *ptr) {
  bpf_trace_printk("%lx\n", *ptr);
```
Signed-off-by: Brenden Blanco <bblanco@plumgrid.com>

Initial implementation of P4->ebpf compiler.

Add contribution section

Use GNUInstallDirs for LIBDIR variable

This should be supported in older versions of cmake, but haven't
explicitly tested those.

Fixes: #243
Signed-off-by: Brenden Blanco <bblanco@plumgrid.com>

biolatency, funclatency, and bpf_log2l usage

add bpf_log2 functions

killsnoop and some minor fixes

opensnoop

Add a better test case for nested probe reads

Signed-off-by: Brenden Blanco <bblanco@plumgrid.com>

Add support for static helper functions