Indicate tshark can read (-r) from stdin and named pipes now.

(Though it works only with certain formats.)  As requested in:
https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=2874#c4

Change-Id: I2ce0509d8750b21ae0167d12459074ba0f9506f8
Reviewed-on: https://code.wireshark.org/review/2646
Reviewed-by: Evan Huus <eapache@gmail.com>
(cherry picked from commit e0461ecd4384377042fe3787acddfcc4d1ed7587)
Reviewed-on: https://code.wireshark.org/review/3989
Reviewed-by: Jeff Morriss <jeff.morriss.ws@gmail.com>