Add support for obtaining the capacity of disks. Add comment while here.

Approved by:	re (glebius)
Fix a minor bug; if strings.subr was included by itself it may not have
access to f_dprintf from common.subr -- fix this by including common.subr.

Approved by:	re (glebius)
Add a USE_DIALOG global (analogous to USE_XDIALOG); useful for inserting
text into a prompt or command syntax iff dialog(1) is in-use.

Approved by:	re (glebius)