userspace/libsinsp/filter: make next_operand return vector<char>

Modify sinsp_filter::next_operand to handle operand which contains
the NUL terminator.
userspace/libsinsp/filter*: modify parse_filter_value to accept len

When the filter value is binary, we need to pass the length of the
value.
userspace/libsinsp/filter*: extend rawval_to_string to accept PT_BYTEBUF
userspace/libsinsp/filter*: evt.buffer return raw_value when m_is_compare
userspace/libsinsp/filterchecks: some refactoring
userspace/libsinsp/filter*: transform evt.buffer in PT_BYTEBUF
userspace/libsinsp: remove old code
Merge branch 'master' into filter-binary-buffer

Conflicts:
	userspace/libsinsp/filter.cpp
Merge pull request #61 from ret2libc/filter-binary-buffer

add the ability to specify binary buffers in filters