sig_analog: Allow three-way flash to time out to silence.

sig_analog: Allow three-way flash to time out to silence.

sig_analog allows users to flash and use the three-way dial
tone as a primitive hold function, simply by never timing
it out.

Some systems allow this dial tone to time out to silence,
so the user is not annoyed by a persistent dial tone.
This option allows the dial tone to time out normally to
silence.

ASTERISK-30004 #close
Resolves: #205

UserNote: The threewaysilenthold option now allows the three-way
dial tone to time out to silence, rather than continuing forever.

(cherry picked from commit 5516763e57d00e4c50be61ac0029fcd903bd10ee)