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 SYSV
• unifdef __osf__
• unifdef hpux
• unifdef SVR4
• unifdef att
• unifdef QNX4
• Use autoconf to check for <paths.h> instead of ifdef BSD
• Use autoconf to check for <protocols/rwhod.h> instead of OS-specific ifdefs
• unifdef __bsdi__
• Use imdent to realign remaining pre-processer directives