Fix loading of lumps from directory on OS X

Use POSIX-compliant opendir() / readdir() functions instead fts_open() / fts_read()
Unlike Linux version, on OS X fts_read() inserts extra slash character between source directory and traversed entry paths
Added GetActorPowerupTics ACS function
Merge branch 'master' of https://github.com/Edward850/zdoom
Merge branch 'master' of https://github.com/rheit/zdoom