Log Message:
Update misc/dvtm to 0.15.

pkgsrc changes:
 * Delete NO_CONFIGURE and convert SUBST_STAGE.* to pre-configure as
   suggested in The pkgsrc guide. (no functional changes intended)

Changes:
 * More complicated default key bindings for potentially destructive
   actions to minimize the chance of accidental presses:

    MOD-x-x closes the focused window
    MOD-q-q quits dvtm

 * Simplified client creation code, elimination of useless intermediate
   shell. This should fix Mod-C (the creation of a new window with the
   same working directory as the currently focused window) for systems
   where procfs is available.
 * Copymode coloring: if enabled in config.def.h (which is currently only
   the case for less) the piped scrollback buffer will contain escape
   codes to preserve the coloring.

    $ DVTM_EDITOR=less dvtm

   Generate some colored output and press MOD-e (i.e. CTRL-g-e by
   default) less should now display the same content (that is with
   color) and provide search functionality etc.
 * Urgent flag indications (after a client received a bell) are redrawn
   as soon as possible.
 * Small bug fixes for terminal emulation and mouse actions.
   Slight cleanup of config.def.h and the dvtm-status script.