libr/bin/format/mach0: check Mach0 format correctness

if there is LC_MAIN or LC_UNIXTHREAD in the file, you can't have other
threads
libr/bin/format/mach0: fix issue with entry offset calculation

mach-o/loader.h: comment flavor to follow Apple header
libr/bin/format/mach0: fix baddr
libr/bin/format/mach0: set entrypoint only on first LC_THREAD
libr/bin/format/mach0: name consistency
libr/bin/format/mach0: correctly retrieve entrypoint
libr/bin/format/mach0: check filetype in r_bin_mach0_get_baddr
libr/bin/format/mach0: remove unused function
libr/bin/format/mach0: find directly main when bin uses LC_MAIN
libr/bin/format/mach0: set main_cmd on the first LC_THREAD

If there are only LC_THREAD, we get the entrypoint from the first
load_command found.