Fix installation of manpages with WITHOUT_COMPRESS broken since r284685.

This partially reverts r284685. An attempt was made in r285295 to fix this
but was not enough. There were still $${} vars in the code that should have
been using the ${_page} and ${_sect} vars, but the bigger problem was that
.for cannot be used on .ALLSRC as it is not defined when the .for is evaluated.

Using ${MAN} here in a .for loop doesn't work either as the paths are not
expanded right for lib/libc/ subdirs despite having a .PATH set for all
of them.

Add some comments around long .else and .endif as well.

Sponsored by: EMC / Isilon Storage Division