Call avformat_network_init/deinit in the programs

Call avformat_network_init/deinit in the programs

Calling the init function will become mandatory at some later
point. By calling it, more heavy network init (such as SSL/TLS
library init) can be done once at startup, instead of implicitly
when used (which could lead to it being done a number of times).

Signed-off-by: Martin Storsjö <martin@martin.st>