Adding a startup (or ramp) time to independent voltage and current sources.

By adding a flaG 'startup' to the 'tran' command, all independent current
and voltage source outputs are scaled by the factor time/StartupTime if
simulation time is smaller than StartupTime. Thus they are ramped up
linearily between times 0 and StartupTime.

The default value of 20us may be overridden by adding a line
set startup_time = <value>
to .spiceinit (or to the top of a .control script), where <value> may
be any valid ngspice real number representing time. Negativ or 0 entries
are ignored.

Code changes include introducing a new mode flag MODESTARTUP and the
scaling in vsrcload.c and isrcload.c.