libr/bin/format/mach0: use segment instead of sections

sections are optional. it should be better to rely on segments
libr/format/mach0: 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.
libr/bin/format/mach0: remove MACH0_ prefix from static functions
libr/bin/format/mach0: fix a mistake. we should check segs, not sects
libr/bin/format/mach0: get right entrypoint also for LC_THREAD
libr/bin/format/mach0: 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