shell: renaming `isnumber` to `is_number` in sc_net_if.c

fix name collision on OS X by renaming
net: handle macro redefinitions for OS X

undefine macros and reorder header includes to get rid of system
defined macros on OS X

fixes #1165
Merge pull request #1173 from thomaseichinger/fix_rpl_udp_osx

native: net: OSX: Fix rpl_udp example