ifdef cleanup

Get rid of a lot of unnecessary ifdefs that just make the code harder to understand and aren't used on the platforms we support today.

• unifdef __UNIXOS2__
• unifdef __QNX__
• unifdef __OSF1__
• Remove ifdefs for non-Solaris SVR4 systems
• Simplify remaining OPEN_MAX logic in FSlibos.h
• unifdef dirty
• Use autoconf-set HAVE_STDINT_H instead of platform-specific ifdefs
• unifdef NCD
• unifdef ISC
• unifdef SUNSYSV