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
More editing flags

PSX-inspired flags to change the render styles of monsters (and other
things).
Less flags, more generic properties
Updated documentation
CANRAISE state definition

CANRAISE state for allowing a monster to be resurrected without an
infinite frame
Merge branch 'master' of https://github.com/rheit/zdoom
Merge branch 'master' of https://github.com/Edward850/zdoom
Merge branch 'master' of https://github.com/rheit/zdoom
Merge branch 'master' into GLEW_VERSION
- got rid of a lot of stuff that's no longer required with GLEW.
- replaced GLUs texture scaling with our own function. This is only used to scale down textures larger than what the hardware can handle so we do not need a dependency to an essentially deprecated library for it.