More work on mach0 format parser

* use segment instead of sections

	sections are optional. it should be better to rely on segments

* remove r_bin_mach0 prefix

	make the source code easier to read. considering this is a plugin, it's
	completely separated from other stuff and we can rename things without
	problems.

* remove MACH0_ prefix from static functions
* fix a mistake. we should check segs, not sects
* get right entrypoint also for LC_THREAD
* fix logic to retrieve entry offset/addr

	first it computes the virtual entrypoint address and then it finds the
	offset in the file through addr_to_offset