sysutils/conky: Add MANPAGES option (on by default)

Since 1.14.0 conky uses python and pandoc to generate its man page.

These make building the whole bunch quite expensive, so add a MANPAGE
option to allow disabling building the man page and wrap in it the
heavy dependencies.

While here:

• Remove some old unneeded reinplace expressions
• Fix one reinplace expression which was not doing what it was meant to

Suggested by: dmgk